diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index 90daaaeae6b..16180a696bc 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -35,7 +35,7 @@ from typing import (
 import anyio
 import websockets
 import websockets.exceptions
-from pydantic import BaseModel, Json, JsonValue
+from pydantic import BaseModel, Json, JsonValue, TypeAdapter
 from typing_extensions import NotRequired, assert_never
 
 from litellm._uuid import uuid
@@ -112,6 +112,7 @@ from litellm.router_utils.add_retry_fallback_headers import (
     get_fallback_errors_from_headers,
     get_hidden_params_dict,
 )
+from litellm.router_utils.routing_groups import parse_routing_groups
 from litellm.types.utils import (
     ModelResponse,
     ModelResponseStream,
@@ -631,6 +632,7 @@ from litellm.types.realtime import RealtimeQueryParams
 from litellm.types.router import (
     DeploymentTypedDict,
     RouterGeneralSettings,
+    RoutingGroup,
     RoutingPlugin,
     SearchToolTypedDict,
     updateDeployment,
@@ -5926,7 +5928,32 @@ class ProxyConfig:
                 combined_router_settings = db_router_settings.param_value
 
             if combined_router_settings:
-                llm_router.update_settings(**combined_router_settings)
+                llm_router.update_settings(**self._drop_invalid_routing_groups(combined_router_settings))
+
+    @staticmethod
+    def _drop_invalid_routing_groups(router_settings: Mapping[str, object]) -> Mapping[str, object]:
+        """
+        A `routing_groups` value already persisted in the DB (saved before
+        save-time validation existed) must not take the rest of the reconcile
+        down with it: log it and apply every other setting, leaving whatever
+        groups the router already holds in place.
+        """
+        raw_groups: Final = router_settings.get("routing_groups")
+        if raw_groups is None:
+            return router_settings
+        try:
+            parse_routing_groups(
+                TypeAdapter(list[RoutingGroup]).validate_python(raw_groups),
+                validate_strategy=Router._validate_routing_strategy,
+            )
+        except ValueError as validation_error:
+            verbose_proxy_logger.error(
+                "Ignoring invalid router_settings.routing_groups from config/DB, all other router settings still "
+                "apply. Fix the routing groups in the Admin UI to load them: %s",
+                validation_error,
+            )
+            return {k: v for k, v in router_settings.items() if k != "routing_groups"}
+        return router_settings
 
     def _add_general_settings_from_db_config(
         self, config_data: dict, general_settings: dict, proxy_logging_obj: ProxyLogging
@@ -14881,6 +14908,15 @@ async def update_config(
         if prisma_client is None:
             raise Exception("No DB Connected")
 
+        if config_info.router_settings is not None:
+            try:
+                parse_routing_groups(
+                    config_info.router_settings.routing_groups,
+                    validate_strategy=Router._validate_routing_strategy,
+                )
+            except ValueError as validation_error:
+                raise HTTPException(status_code=400, detail={"error": str(validation_error)})
+
         async def _read_section(param_name: str) -> dict:
             row: Final = await ConfigRepository(prisma_client).table.find_first(where={"param_name": param_name})
             if row is None or row.param_value is None:
diff --git a/litellm/router.py b/litellm/router.py
index feaf69a44ae..a00b919cf19 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -154,6 +154,7 @@ from litellm.router_utils.router_callbacks.track_deployment_metrics import (
     increment_deployment_failures_for_current_minute,
     increment_deployment_successes_for_current_minute,
 )
+from litellm.router_utils.routing_groups import parse_routing_groups
 from litellm.scheduler import FlowItem, Scheduler
 from litellm.types.llms.openai import (
     AllMessageValues,
@@ -898,7 +899,8 @@ class Router:
             return strategy.value
         return strategy
 
-    def _validate_routing_strategy(self, routing_strategy: RoutingStrategy | str | None) -> None:
+    @staticmethod
+    def _validate_routing_strategy(routing_strategy: RoutingStrategy | str | None) -> None:
         # See: https://github.com/BerriAI/litellm/issues/11330
         valid_strategy_strings: Final = ["simple-shuffle", "lar1"] + [s.value for s in RoutingStrategy]
         if routing_strategy is None:
@@ -1014,66 +1016,42 @@ class Router:
         at most one explicit group. Constructs per-group strategy selectors so
         groups with different `routing_strategy_args` track independent state.
 
+        Validation runs to completion before any router state changes, so an
+        invalid input raises with the previously loaded groups left intact.
+
         Models not claimed by any explicit group are served by the implicit
         `"default"` group, whose selectors are the `self.<strategy>_logger`
         attributes set up in `routing_strategy_init`.
         """
+        known_model_names: Final = frozenset(m["model_name"] for m in (self.model_list or []) if m.get("model_name"))
+        groups: Final = parse_routing_groups(
+            groups_input,
+            validate_strategy=self._validate_routing_strategy,
+            known_model_names=known_model_names,
+        )
+
         self._unregister_router_selectors(
             [sel for selectors in getattr(self, "_group_selectors", {}).values() for sel in selectors.values()]
         )
 
-        self._routing_groups: dict[str, RoutingGroup] = {}
-        self._model_to_group: dict[str, str] = {}
-        self._group_selectors: dict[str, dict[str, Any]] = {}
-
-        if not groups_input:
-            return
-
-        known_model_names: Final = {m.get("model_name") for m in (self.model_list or []) if m.get("model_name")}
-
-        seen_group_names: Final[set] = set()
-        for raw in groups_input:
-            group = raw if isinstance(raw, RoutingGroup) else RoutingGroup(**raw)
-
-            if not group.group_name:
-                raise ValueError("routing_groups: group_name must be non-empty.")
-            if group.group_name == "default":
-                raise ValueError("routing_groups: 'default' is reserved for the implicit fallback group.")
-            if group.group_name in seen_group_names:
-                raise ValueError(
-                    f"routing_groups: group names must be unique, duplicate group_name '{group.group_name}'."
+        self._routing_groups: dict[str, RoutingGroup] = {group.group_name: group for group in groups}
+        self._model_to_group: dict[str, str] = {
+            model_name: group.group_name for group in groups for model_name in group.models
+        }
+        self._group_selectors: dict[str, dict[str, Any]] = {
+            group.group_name: (
+                {}
+                if (
+                    selector := self._build_strategy_selector(
+                        strategy=group.routing_strategy,
+                        routing_strategy_args=group.routing_strategy_args or {},
+                    )
                 )
-            seen_group_names.add(group.group_name)
-
-            self._validate_routing_strategy(group.routing_strategy)
-
-            for model_name in group.models:
-                if model_name in self._model_to_group:
-                    raise ValueError(
-                        f"routing_groups: model_name '{model_name}' appears in "
-                        f"both '{self._model_to_group[model_name]}' and "
-                        f"'{group.group_name}'. Each model may belong to at most one group."
-                    )
-                if known_model_names and model_name not in known_model_names:
-                    verbose_router_logger.warning(
-                        "routing_groups: model_name '%s' (group '%s') is not in model_list; "
-                        "the group entry will only take effect once a deployment with that "
-                        "model_name is added.",
-                        model_name,
-                        group.group_name,
-                    )
-                self._model_to_group[model_name] = group.group_name
-
-            self._routing_groups[group.group_name] = group
-
-            strategy_value = self._normalize_strategy(group.routing_strategy) or ""
-            group_selector = self._build_strategy_selector(
-                strategy=group.routing_strategy,
-                routing_strategy_args=group.routing_strategy_args or {},
-            )
-            self._group_selectors[group.group_name] = (
-                {strategy_value: group_selector} if group_selector is not None else {}
+                is None
+                else {self._normalize_strategy(group.routing_strategy) or "": selector}
             )
+            for group in groups
+        }
 
     _OVERRIDABLE_ROUTING_STRATEGIES: frozenset[str] = frozenset({"simple-shuffle", *_DEFAULT_SELECTOR_ATTR_BY_STRATEGY})
 
diff --git a/litellm/router_utils/routing_groups.py b/litellm/router_utils/routing_groups.py
new file mode 100644
index 00000000000..4b3f20c5b72
--- /dev/null
+++ b/litellm/router_utils/routing_groups.py
@@ -0,0 +1,77 @@
+"""
+Validation for `router_settings.routing_groups`, shared by the Router and the
+proxy's config-update endpoint so a config the UI saves cannot be one the
+runtime refuses to load.
+"""
+
+from collections.abc import Callable, Sequence
+from typing import Final
+
+from litellm._logging import verbose_router_logger
+from litellm.types.router import RoutingGroup, RoutingStrategy
+
+ValidateStrategy = Callable[[RoutingStrategy | str | None], None]
+
+
+def parse_routing_groups(
+    groups_input: Sequence[RoutingGroup | dict] | None,
+    validate_strategy: ValidateStrategy,
+    known_model_names: frozenset[str] = frozenset(),
+) -> tuple[RoutingGroup, ...]:
+    """
+    Parses and validates `routing_groups`, raising `ValueError` on the first
+    problem found. Every check runs before the caller mutates any state, so an
+    invalid update can never leave a router holding a half-applied set of
+    groups.
+    """
+    if not groups_input:
+        return ()
+
+    groups: Final = tuple(raw if isinstance(raw, RoutingGroup) else RoutingGroup(**raw) for raw in groups_input)
+
+    if any(not group.group_name for group in groups):
+        raise ValueError("routing_groups: group_name must be non-empty.")
+
+    if any(group.group_name == "default" for group in groups):
+        raise ValueError("routing_groups: 'default' is reserved for the implicit fallback group.")
+
+    names: Final = [group.group_name for group in groups]
+    duplicate_names: Final = sorted({name for name in names if names.count(name) > 1})
+    if duplicate_names:
+        raise ValueError(f"routing_groups: group names must be unique, duplicate group_name '{duplicate_names[0]}'.")
+
+    for group in groups:
+        validate_strategy(group.routing_strategy)
+
+    owners_by_model: Final = {
+        model_name: tuple(group.group_name for group in groups if model_name in group.models)
+        for model_name in dict.fromkeys(model_name for group in groups for model_name in group.models)
+    }
+    conflicts: Final = tuple(
+        f"model_name '{model_name}' appears in {' and '.join(repr(owner) for owner in owners)}"
+        for model_name, owners in owners_by_model.items()
+        if len(owners) > 1
+    )
+    if conflicts:
+        raise ValueError(f"routing_groups: {'; '.join(conflicts)}. Each model may belong to at most one group.")
+
+    unknown_models: Final = (
+        tuple(
+            (model_name, group.group_name)
+            for group in groups
+            for model_name in group.models
+            if model_name not in known_model_names
+        )
+        if known_model_names
+        else ()
+    )
+    for model_name, group_name in unknown_models:
+        verbose_router_logger.warning(
+            "routing_groups: model_name '%s' (group '%s') is not in model_list; "
+            "the group entry will only take effect once a deployment with that "
+            "model_name is added.",
+            model_name,
+            group_name,
+        )
+
+    return groups
diff --git a/tests/test_litellm/proxy/test_proxy_server.py b/tests/test_litellm/proxy/test_proxy_server.py
index 580a58885d9..98e29b3e48c 100644
--- a/tests/test_litellm/proxy/test_proxy_server.py
+++ b/tests/test_litellm/proxy/test_proxy_server.py
@@ -11278,3 +11278,24 @@ async def test_setup_prisma_client_returns_none_when_connect_itself_fails(monkey
     assert result is None
     assert mock_client.start_db_health_watchdog_task.await_count == 0
     assert mock_client.health_check.await_count == 0
+
+
+def test_drop_invalid_routing_groups_keeps_other_router_settings():
+    from litellm.proxy.proxy_server import ProxyConfig
+
+    valid = {
+        "num_retries": 3,
+        "routing_groups": [
+            {"group_name": "g1", "models": ["m1"], "routing_strategy": "least-busy"},
+        ],
+    }
+    assert ProxyConfig._drop_invalid_routing_groups(valid) == valid
+
+    overlapping = {
+        "num_retries": 3,
+        "routing_groups": [
+            {"group_name": "g1", "models": ["m1"], "routing_strategy": "least-busy"},
+            {"group_name": "g2", "models": ["m1"], "routing_strategy": "least-busy"},
+        ],
+    }
+    assert ProxyConfig._drop_invalid_routing_groups(overlapping) == {"num_retries": 3}
diff --git a/tests/test_litellm/router_strategy/test_router_routing_groups.py b/tests/test_litellm/router_strategy/test_router_routing_groups.py
index b8dcdacd8a3..8865bdf0172 100644
--- a/tests/test_litellm/router_strategy/test_router_routing_groups.py
+++ b/tests/test_litellm/router_strategy/test_router_routing_groups.py
@@ -726,3 +726,65 @@ def test_strategy_reinit_unregisters_override_selectors():
     assert router._override_selectors == {}
     assert not any(id(cb) == id(override_selector) for cb in litellm.callbacks)
     assert router._get_override_strategy_selector("latency-based-routing") is router.lowestlatency_logger
+
+
+def test_failed_routing_groups_update_keeps_previous_groups():
+    router = _build_router(
+        routing_groups=[
+            {"group_name": "g1", "models": ["filtered-model"], "routing_strategy": "latency-based-routing"},
+        ],
+    )
+    selector = router._group_selectors["g1"]["latency-based-routing"]
+
+    with pytest.raises(ValueError, match="appears in"):
+        router.update_settings(
+            routing_groups=[
+                {"group_name": "g1", "models": ["filtered-model"], "routing_strategy": "latency-based-routing"},
+                {"group_name": "g2", "models": ["filtered-model"], "routing_strategy": "least-busy"},
+            ],
+        )
+
+    assert list(router._routing_groups) == ["g1"]
+    assert router._model_to_group == {"filtered-model": "g1"}
+    assert router._group_selectors["g1"]["latency-based-routing"] is selector
+    assert router._get_routing_context("filtered-model", None) == ("latency-based-routing", selector)
+
+
+def test_overlap_error_names_every_conflicting_model():
+    with pytest.raises(ValueError) as exc_info:
+        _build_router(
+            routing_groups=[
+                {
+                    "group_name": "g1",
+                    "models": ["filtered-model", "other-model"],
+                    "routing_strategy": "latency-based-routing",
+                },
+                {
+                    "group_name": "g2",
+                    "models": ["filtered-model", "other-model"],
+                    "routing_strategy": "least-busy",
+                },
+            ],
+        )
+    message = str(exc_info.value)
+    assert "'filtered-model' appears in 'g1' and 'g2'" in message
+    assert "'other-model' appears in 'g1' and 'g2'" in message
+
+
+def test_invalid_group_strategy_does_not_leak_a_selector():
+    router = _build_router(
+        routing_groups=[
+            {"group_name": "g1", "models": ["filtered-model"], "routing_strategy": "latency-based-routing"},
+        ],
+    )
+    selector = router._group_selectors["g1"]["latency-based-routing"]
+
+    with pytest.raises(ValueError, match="Invalid routing_strategy"):
+        router.update_settings(
+            routing_groups=[
+                {"group_name": "g2", "models": ["other-model"], "routing_strategy": "not-a-real-strategy"},
+            ],
+        )
+
+    assert list(router._routing_groups) == ["g1"]
+    assert any(id(cb) == id(selector) for cb in litellm.callbacks)
diff --git a/ui/litellm-dashboard/src/components/routing_groups/RoutingGroupModal.tsx b/ui/litellm-dashboard/src/components/routing_groups/RoutingGroupModal.tsx
index 5b7c0fce6ef..b124217b2fb 100644
--- a/ui/litellm-dashboard/src/components/routing_groups/RoutingGroupModal.tsx
+++ b/ui/litellm-dashboard/src/components/routing_groups/RoutingGroupModal.tsx
@@ -3,6 +3,7 @@
 import React, { useMemo } from "react";
 import { Form, Input, Modal, Select, Space, Typography } from "antd";
 import type { RoutingGroup, RoutingStrategy } from "./types";
+import { modelConflictError } from "./modelOwnership";
 
 const { Text, Paragraph } = Typography;
 
@@ -14,6 +15,7 @@ interface RoutingGroupModalProps {
   strategyDescriptions: Record<string, string>;
   modelOptions: string[];
   existingGroupNames: string[];
+  groupNameByModel: Record<string, string>;
   onClose: () => void;
   onSubmit: (group: RoutingGroup) => Promise<void> | void;
   saving?: boolean;
@@ -31,6 +33,16 @@ const STRATEGIES_WITH_ARGS = new Set<string>(["latency-based-routing", "usage-ba
 const GROUP_NAME_PATTERN = /^[A-Za-z0-9._-]+$/;
 const GROUP_NAME_MAX_LENGTH = 64;
 
+const modelRules = (groupNameByModel: Record<string, string>) => [
+  { required: true, message: "Select at least one model" },
+  {
+    validator: (_: unknown, value: string[] | undefined) => {
+      const error = modelConflictError(value, groupNameByModel);
+      return error ? Promise.reject(new Error(error)) : Promise.resolve();
+    },
+  },
+];
+
 const RoutingGroupModal: React.FC<RoutingGroupModalProps> = ({
   open,
   mode,
@@ -39,6 +51,7 @@ const RoutingGroupModal: React.FC<RoutingGroupModalProps> = ({
   strategyDescriptions,
   modelOptions,
   existingGroupNames,
+  groupNameByModel,
   onClose,
   onSubmit,
   saving,
@@ -125,7 +138,7 @@ const RoutingGroupModal: React.FC<RoutingGroupModalProps> = ({
               },
             },
           ]}
-          extra="Use this name as the model in API calls — LiteLLM routes the request to one of the group's models."
+          extra="Names the group's shared routing strategy. Requests still use the model names, not this name."
         >
           <Input placeholder="fast-chat" disabled={mode === "edit"} />
         </Form.Item>
@@ -133,8 +146,8 @@ const RoutingGroupModal: React.FC<RoutingGroupModalProps> = ({
         <Form.Item
           label="Models"
           name="models"
-          rules={[{ required: true, message: "Select at least one model" }]}
-          extra="Models from your model list that this group routes between."
+          rules={modelRules(groupNameByModel)}
+          extra="Models from your model list that this group routes between. A model can only be in one group."
         >
           <Select
             mode="multiple"
diff --git a/ui/litellm-dashboard/src/components/routing_groups/index.tsx b/ui/litellm-dashboard/src/components/routing_groups/index.tsx
index 8b33b050377..2aadba032ec 100644
--- a/ui/litellm-dashboard/src/components/routing_groups/index.tsx
+++ b/ui/litellm-dashboard/src/components/routing_groups/index.tsx
@@ -11,6 +11,7 @@ import useProxySettings from "@/app/(dashboard)/hooks/proxySettings/useProxySett
 import RoutingGroupsTable from "./RoutingGroupsTable";
 import RoutingGroupModal from "./RoutingGroupModal";
 import NotificationsManager from "../molecules/notifications_manager";
+import { groupNameByModel } from "./modelOwnership";
 import type { RoutingGroup } from "./types";
 
 const { Text } = Typography;
@@ -56,6 +57,8 @@ const RoutingGroups: React.FC = () => {
     return Array.from(new Set(names));
   }, [modelHub]);
 
+  const ownerByModel = groupNameByModel(groups, drawerMode === "edit" ? editingGroup?.group_name : undefined);
+
   const openCreate = () => {
     setDrawerMode("create");
     setEditingGroup(null);
@@ -141,6 +144,7 @@ const RoutingGroups: React.FC = () => {
         strategyDescriptions={strategyDescriptions}
         modelOptions={modelOptions}
         existingGroupNames={groups.map((g) => g.group_name)}
+        groupNameByModel={ownerByModel}
         onClose={() => setDrawerOpen(false)}
         onSubmit={handleSubmit}
         saving={saveMutation.isPending}
diff --git a/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.test.ts b/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.test.ts
new file mode 100644
index 00000000000..b1fea46d6f0
--- /dev/null
+++ b/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.test.ts
@@ -0,0 +1,29 @@
+import { groupNameByModel, modelConflictError } from "./modelOwnership";
+import type { RoutingGroup } from "./types";
+
+const groups: RoutingGroup[] = [
+  { group_name: "cheap", models: ["m1", "m2"], routing_strategy: "latency-based-routing" },
+  { group_name: "security", models: ["m3"], routing_strategy: "least-busy" },
+];
+
+describe("groupNameByModel", () => {
+  it("maps every claimed model to its owning group", () => {
+    expect(groupNameByModel(groups)).toEqual({ m1: "cheap", m2: "cheap", m3: "security" });
+  });
+
+  it("excludes the group being edited so its own models stay selectable", () => {
+    expect(groupNameByModel(groups, "cheap")).toEqual({ m3: "security" });
+  });
+});
+
+describe("modelConflictError", () => {
+  it("passes models that no other group claims", () => {
+    expect(modelConflictError(["m4"], groupNameByModel(groups, "cheap"))).toBeNull();
+    expect(modelConflictError(undefined, groupNameByModel(groups))).toBeNull();
+  });
+
+  it("names every model already claimed by another group", () => {
+    const error = modelConflictError(["m1", "m3", "m4"], groupNameByModel(groups));
+    expect(error).toBe('Each model may belong to at most one group. Already claimed: m1 (in "cheap"), m3 (in "security")');
+  });
+});
diff --git a/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.ts b/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.ts
new file mode 100644
index 00000000000..c67ae77d066
--- /dev/null
+++ b/ui/litellm-dashboard/src/components/routing_groups/modelOwnership.ts
@@ -0,0 +1,18 @@
+import type { RoutingGroup } from "./types";
+
+export const groupNameByModel = (groups: RoutingGroup[], excludeGroupName?: string): Record<string, string> =>
+  Object.fromEntries(
+    groups
+      .filter((group) => group.group_name !== excludeGroupName)
+      .flatMap((group) => group.models.map((model) => [model, group.group_name] as const)),
+  );
+
+export const modelConflictError = (
+  models: string[] | undefined,
+  ownerByModel: Record<string, string>,
+): string | null => {
+  const conflicts = (models ?? []).filter((model) => ownerByModel[model] !== undefined);
+  if (conflicts.length === 0) return null;
+  const detail = conflicts.map((model) => `${model} (in "${ownerByModel[model]}")`).join(", ");
+  return `Each model may belong to at most one group. Already claimed: ${detail}`;
+};