diff --git a/chrome/content/zotero/integration/citationDialog.js b/chrome/content/zotero/integration/citationDialog.js
index 59aa715198..6014e1fc31 100644
--- a/chrome/content/zotero/integration/citationDialog.js
+++ b/chrome/content/zotero/integration/citationDialog.js
@@ -218,6 +218,10 @@ function cleanupBeforeDialogClosing() {
 	allParams[currentLayout.type] = params;
 	Zotero.Prefs.set("integration.citationDialog.windowParams", JSON.stringify(allParams));
 
+	if (libraryLayout.collectionTreeWidth) {
+		Zotero.Prefs.set("integration.citationDialog.collectionTreeWidth", libraryLayout.collectionTreeWidth);
+	}
+
 	// Only library mode in annotations dialog
 	if (!DIALOG_STATE.isAddingAnnotations()) {
 		Zotero.Prefs.set("integration.citationDialogLastUsedMode", currentLayout.type);
@@ -523,6 +527,7 @@ class LibraryLayout extends Layout {
 		// on mouse scrollwheel in suggested items, scroll the list horizontally
 		_id("library-other-items").addEventListener('wheel', this._scrollHorizontallyOnWheel);
 		this._initSidepane();
+		this._initCollectionsTreeDivider();
 	}
 
 	// Create item node for an item group and store item ids in itemIDs attribute
@@ -823,6 +828,53 @@ class LibraryLayout extends Layout {
 			_id("annotations-sidebar-filter").dispatchEvent(new Event('input', { bubbles: true }));
 		});
 	}
+
+	// Draggable divider to resize collectionTree
+	_initCollectionsTreeDivider() {
+		let setCollectionTreeWidth = (width) => {
+			let minWidth = 200;
+			let maxWidth = Math.floor(window.innerWidth * 0.4); // 40% of window width
+			let nextWidth = Math.round(Math.max(minWidth, Math.min(width, maxWidth))); // enforce width between 200px and maxWidth
+			_id("collections-tree-container").style.width = `${nextWidth}px`;
+			this.collectionTreeWidth = nextWidth;
+		};
+
+		let divider = _id("collections-tree-divider");
+		let savedWidth = Zotero.Prefs.get("integration.citationDialog.collectionTreeWidth");
+		setCollectionTreeWidth(savedWidth || 200);
+
+		let startX = 0;
+		let startWidth = 0;
+		let onPointerMove = (event) => {
+			let delta = event.clientX - startX;
+			if (Zotero.rtl) delta = -delta;
+			setCollectionTreeWidth(startWidth + delta);
+		};
+		let onPointerUp = (event) => {
+			divider.removeEventListener("pointermove", onPointerMove);
+			divider.removeEventListener("pointerup", onPointerUp);
+			divider.removeEventListener("pointercancel", onPointerUp);
+			try {
+				divider.releasePointerCapture(event.pointerId);
+			}
+			catch (e) {}
+		};
+		divider.addEventListener("pointerdown", (event) => {
+			if (event.button !== 0) return;
+			event.preventDefault();
+			startX = event.clientX;
+			startWidth = _id("collections-tree-container").getBoundingClientRect().width;
+			divider.setPointerCapture(event.pointerId);
+			divider.addEventListener("pointermove", onPointerMove);
+			divider.addEventListener("pointerup", onPointerUp);
+			divider.addEventListener("pointercancel", onPointerUp);
+		});
+
+		// Update max width when the window is resized
+		window.addEventListener("resize", () => {
+			setCollectionTreeWidth(this.collectionTreeWidth);
+		});
+	}
 	
 	async _onCollectionSelection() {
 		var collectionTreeRow = this.collectionsView.getRow(this.collectionsView.selection.focused);
diff --git a/chrome/content/zotero/integration/citationDialog.xhtml b/chrome/content/zotero/integration/citationDialog.xhtml
index e847afce4b..c6d039a22e 100644
--- a/chrome/content/zotero/integration/citationDialog.xhtml
+++ b/chrome/content/zotero/integration/citationDialog.xhtml
@@ -78,10 +78,11 @@
 				<span id="library-no-suggested-items-message"/>
 			</div>
 			<div class="divider"></div>
-			<div id="library-trees" class="hbox">			
+			<div id="library-trees" class="hbox">
 				<div id="collections-tree-container" class="vbox virtualized-table-container">
 					<div id="zotero-collections-tree" data-tabindex="50"/>
 				</div>
+				<div id="collections-tree-divider"/>
 				<div id="item-tree-container" class="hbox virtualized-table-container">
 					<div id="zotero-items-tree" data-tabindex="60" style="--highlight-color: var(--accent-blue30);"/>
 				</div>
diff --git a/scss/components/_citationDialog.scss b/scss/components/_citationDialog.scss
index 87bfe5a5d8..df915ef9f3 100644
--- a/scss/components/_citationDialog.scss
+++ b/scss/components/_citationDialog.scss
@@ -353,7 +353,8 @@
 			#collections-tree-container {
 				min-width: 200px;
 				min-height: 100%;
-				border-inline-end: var(--material-panedivider);
+				width: 200px;
+				flex-shrink: 0;
 				& > #zotero-collections-tree {
 					background: var(--material-sidepane) !important;
 				}
@@ -361,6 +362,25 @@
 					padding-top: 8px;
 				}
 			}
+			
+			#collections-tree-divider {
+				flex: 0 0 9px;
+				margin-inline-end: -8px;
+				position: relative;
+				cursor: ew-resize;
+				z-index: 1;
+				-moz-window-dragging: no-drag;
+
+				&::before {
+					content: "";
+					position: absolute;
+					top: 0;
+					bottom: 0;
+					inset-inline-start: 0;
+					width: 1px;
+					background-color: var(--color-panedivider);
+				}
+			}
 		
 			#item-tree-container {
 				min-height: 100%;