diff --git a/chrome/content/zotero/xpcom/dictionaries.js b/chrome/content/zotero/xpcom/dictionaries.js
new file mode 100644
index 0000000000..8b96ae6f0d
--- /dev/null
+++ b/chrome/content/zotero/xpcom/dictionaries.js
@@ -0,0 +1,248 @@
+/*
+    ***** BEGIN LICENSE BLOCK *****
+    
+    Copyright © 2021 Corporation for Digital Scholarship
+                     Vienna, Virginia, USA
+                     http://digitalscholar.org/
+    
+    This file is part of Zotero.
+    
+    Zotero is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+    
+    Zotero is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Affero General Public License for more details.
+    
+    You should have received a copy of the GNU Affero General Public License
+    along with Zotero.  If not, see <http://www.gnu.org/licenses/>.
+    
+    ***** END LICENSE BLOCK *****
+*/
+
+Zotero.Dictionaries = new function () {
+	let _dictionaries = [];
+	let _spellChecker = Cc['@mozilla.org/spellchecker/engine;1']
+		.getService(Ci.mozISpellCheckingEngine);
+	_spellChecker.QueryInterface(Ci.mozISpellCheckingEngine);
+
+	Zotero.defineProperty(this, 'baseURL', {
+		get: () => {
+			let url = ZOTERO_CONFIG.DICTIONARIES_URL;
+			if (!url.endsWith('/')) {
+				url += '/';
+			}
+			return url;
+		}
+	});
+	
+	Zotero.defineProperty(this, 'dictionaries', {
+		get: () => {
+			return _dictionaries;
+		}
+	});
+
+	/**
+	 * Load all dictionaries
+	 *
+	 * @return {Promise}
+	 */
+	this.init = async function () {
+		let dictionariesDir = OS.Path.join(Zotero.Profile.dir, 'dictionaries');
+		if (!(await OS.File.exists(dictionariesDir))) {
+			return;
+		}
+		let iterator = new OS.File.DirectoryIterator(dictionariesDir);
+		try {
+			await iterator.forEach(async function (entry) {
+				if (entry.name.startsWith('.')) {
+					return;
+				}
+				try {
+					let dir = OS.Path.join(dictionariesDir, entry.name);
+					await _loadDictionary(dir);
+				}
+				catch (e) {
+					Zotero.logError(e);
+				}
+			});
+		}
+		finally {
+			iterator.close();
+		}
+	};
+
+	/**
+	 * Get available dictionaries from server
+	 *
+	 * @return {Promise<Object>}
+	 */
+	this.fetchDictionariesList = async function () {
+		let url = this.baseURL + 'dictionaries.json';
+		let req = await Zotero.HTTP.request('GET', url, { responseType: 'json' });
+		return req.response;
+	};
+
+	/**
+	 * Install the most popular dictionary for specified locale
+	 *
+	 * @param locale
+	 * @return {Promise<Boolean>}
+	 */
+	this.installByLocale = async function (locale) {
+		let dictionaries = await this.fetchDictionariesList();
+		let matched = dictionaries.filter(x => x.locale === locale);
+		if (!matched.length) {
+			matched = dictionaries.filter(x => x.locale === locale.split(/[-_]/)[0]);
+		}
+		if (!matched.length) {
+			return false;
+		}
+		matched.sort((a, b) => b.users - a.users);
+		await this.install(matched[0].id);
+		return true;
+	};
+
+	/**
+	 * Remove all dictionaries targeting specific locale
+	 *
+	 * @param locale
+	 * @return {Promise}
+	 */
+	this.removeByLocale = async function(locale) {
+		for (let dictionary of _dictionaries) {
+			if (dictionary.locales.includes(locale)
+			|| dictionary.locales.some(x => x === locale.split(/[-_]/)[0])) {
+				await this.remove(dictionary.id);
+			}
+		}
+	};
+
+	/**
+	 * Install dictionary by extension id,
+	 * e.g., `en-NZ@dictionaries.addons.mozilla.org`
+	 *
+	 * @param {String} id - Dictionary extension id
+	 * @return {Promise}
+	 */
+	this.install = async function (id) {
+		await this.remove(id);
+		let url = this.baseURL + id + '.xpi';
+		let xpiPath = OS.Path.join(Zotero.getTempDirectory().path, id);
+		let dir = OS.Path.join(Zotero.Profile.dir, 'dictionaries', id);
+		let zipReader = Components.classes['@mozilla.org/libjar/zip-reader;1']
+			.createInstance(Components.interfaces.nsIZipReader);
+		try {
+			await Zotero.File.download(url, xpiPath);
+
+			zipReader.open(Zotero.File.pathToFile(xpiPath));
+			zipReader.test(null);
+
+			// Create directories
+			let entries = zipReader.findEntries('*/');
+			while (entries.hasMore()) {
+				let entry = entries.getNext();
+				let destPath = OS.Path.join(dir, entry);
+				await Zotero.File.createDirectoryIfMissingAsync(destPath, { from: Zotero.Profile.dir });
+			}
+
+			// Extract files
+			entries = zipReader.findEntries('*');
+			while (entries.hasMore()) {
+				let entry = entries.getNext();
+				if (entry.substr(-1) === '/') {
+					continue;
+				}
+				let destPath = OS.Path.join(dir, entry);
+				zipReader.extract(entry, Zotero.File.pathToFile(destPath));
+			}
+
+			zipReader.close();
+			await OS.File.remove(xpiPath);
+			await _loadDictionary(dir);
+		}
+		catch (e) {
+			if (await OS.File.exists(xpiPath)) {
+				await OS.File.remove(xpiPath);
+			}
+			if (await OS.File.exists(dir)) {
+				await OS.File.removeDir(dir);
+			}
+			throw e;
+		}
+	};
+
+	/**
+	 * Remove dictionary by extension id
+	 *
+	 * @param {String} id
+	 * @return {Promise}
+	 */
+	this.remove = async function (id) {
+		let dictionaryIndex = _dictionaries.findIndex(x => x.id === id);
+		if (dictionaryIndex !== -1) {
+			let dictionary = _dictionaries[dictionaryIndex];
+			try {
+				let manifestPath = OS.Path.join(dictionary.dir, 'manifest.json');
+				let manifest = await Zotero.File.getContentsAsync(manifestPath);
+				manifest = JSON.parse(manifest);
+				for (let locale in manifest.dictionaries) {
+					let dicPath = manifest.dictionaries[locale];
+					let affPath = OS.Path.join(dictionary.dir, dicPath.slice(0, -3) + 'aff');
+					_spellChecker.removeDictionary(locale, Zotero.File.pathToFile(affPath));
+				}
+			}
+			catch (e) {
+				Zotero.logError(e);
+			}
+			await OS.File.removeDir(dictionary.dir);
+			_dictionaries.splice(dictionaryIndex, 1);
+		}
+	};
+
+	/**
+	 * Update all dictionaries
+	 *
+	 * @return {Promise}
+	 */
+	this.update = async function () {
+		let availableDictionaries = await this.fetchDictionariesList();
+		for (let dictionary of _dictionaries) {
+			let availableDictionary = availableDictionaries.find(x => x.id === dictionary.id);
+			if (availableDictionary && availableDictionary.version > dictionary.version) {
+				await this.install(availableDictionary.id);
+			}
+		}
+	};
+
+	/**
+	 * Load dictionary from specified dir
+	 *
+	 * @param {String} dir
+	 * @return {Promise}
+	 */
+	async function _loadDictionary(dir) {
+		let manifestPath = OS.Path.join(dir, 'manifest.json');
+		let manifest = await Zotero.File.getContentsAsync(manifestPath);
+		manifest = JSON.parse(manifest);
+		let id;
+		if (manifest.applications && manifest.applications.gecko) {
+			id = manifest.applications.gecko.id;
+		}
+		else {
+			id = manifest.browser_specific_settings.gecko.id;
+		}
+		let version = manifest.version;
+		let locales = [];
+		for (let locale in manifest.dictionaries) {
+			locales.push(locale);
+			let dicPath = manifest.dictionaries[locale];
+			let affPath = OS.Path.join(dir, dicPath.slice(0, -3) + 'aff');
+			_spellChecker.addDictionary(locale, Zotero.File.pathToFile(affPath));
+		}
+		_dictionaries.push({ id, locales, version, dir });
+	}
+};
diff --git a/chrome/content/zotero/xpcom/editorInstance.js b/chrome/content/zotero/xpcom/editorInstance.js
index 96ccd4a14d..8d7560bcd4 100644
--- a/chrome/content/zotero/xpcom/editorInstance.js
+++ b/chrome/content/zotero/xpcom/editorInstance.js
@@ -23,6 +23,8 @@
     ***** END LICENSE BLOCK *****
 */
 
+Components.utils.import("resource://gre/modules/InlineSpellChecker.jsm");
+
 // Note: TinyMCE is automatically doing some meaningless corrections to
 // note-editor produced HTML. Which might result to more
 // conflicts, especially in group libraries
@@ -73,7 +75,8 @@ class EditorInstance {
 		});
 		this._prefObserverIDs = [
 			Zotero.Prefs.registerObserver('note.fontSize', this._handleFontChange),
-			Zotero.Prefs.registerObserver('note.fontFamily', this._handleFontChange)
+			Zotero.Prefs.registerObserver('note.fontFamily', this._handleFontChange),
+			Zotero.Prefs.registerObserver('layout.spellcheckDefault', this._handleSpellCheckChange, true)
 		];
 		
 		// Run Cut/Copy/Paste with chrome privileges
@@ -209,6 +212,20 @@ class EditorInstance {
 	_handleFontChange = () => {
 		this._postMessage({ action: 'updateFont', font: this._getFont() });
 	}
+
+	_handleSpellCheckChange = () => {
+		try {
+			let spellChecker = this._getSpellChecker();
+			let value = Zotero.Prefs.get('layout.spellcheckDefault', true);
+			if (!value && spellChecker.enabled
+				|| value && !spellChecker.enabled) {
+				spellChecker.toggleEnabled();
+			}
+		}
+		catch (e) {
+			Zotero.logError(e);
+		}
+	}
 	
 	_showInLibrary(ids) {
 		if (!Array.isArray(ids)) {
@@ -788,9 +805,93 @@ class EditorInstance {
 		}
 		
 		appendItems(this._popup, itemGroups);
+		
+		// TODO: Localize
+		
+		// Spell checker
+		let spellChecker = this._getSpellChecker();
+		
+		// Separator
+		var separator = this._popup.ownerDocument.createElement('menuseparator');
+		this._popup.appendChild(separator);
+		// Check Spelling
+		var menuitem = this._popup.ownerDocument.createElement('menuitem');
+		menuitem.setAttribute('label', 'Check Spelling');
+		menuitem.setAttribute('checked', spellChecker.enabled);
+		menuitem.addEventListener('command', () => {
+			// Possible values: 0 - off, 1 - only multi-line, 2 - multi and single line input boxes
+			Zotero.Prefs.set('layout.spellcheckDefault', spellChecker.enabled ? 0 : 1, true);
+		});
+		this._popup.append(menuitem);
+
+		if (spellChecker.enabled) {
+			// Languages menu
+			var menu = this._popup.ownerDocument.createElement('menu');
+			menu.setAttribute('label', 'Languages');
+			this._popup.append(menu);
+			// Languages menu popup
+			var menupopup = this._popup.ownerDocument.createElement('menupopup');
+			menu.append(menupopup);
+			
+			spellChecker.addDictionaryListToMenu(menupopup, null);
+			
+			for (var menuitem of menupopup.children) {
+				// 'spell-check-dictionary-en-US'
+				let locale = menuitem.id.slice(23);
+				let label = Zotero.Locale.availableLocales[locale];
+				if (!label) {
+					for(let key in Zotero.Locale.availableLocales) {
+						if (key.split('-')[0] === locale) {
+							label = Zotero.Locale.availableLocales[key];
+						}
+					}
+				}
+				
+				if (label) {
+					menuitem.setAttribute('label', label);
+				}
+			}
+			
+			// Separator
+			var separator = this._popup.ownerDocument.createElement('menuseparator');
+			menupopup.appendChild(separator);
+			// Add Dictionaries
+			var menuitem = this._popup.ownerDocument.createElement('menuitem');
+			menuitem.setAttribute('label', 'Add Dictionaries...');
+			menuitem.addEventListener('command', () => {
+				Zotero.Utilities.Internal.openPreferences('zotero-prefpane-advanced');
+			});
+			menupopup.append(menuitem);
+			
+			let selection = this._iframeWindow.getSelection();
+			if (selection) {
+				spellChecker.initFromEvent(
+					selection.anchorNode,
+					selection.anchorOffset
+				);
+			}
+
+			let firstElementChild = this._popup.firstElementChild;
+			let suggestionCount = spellChecker.addSuggestionsToMenu(this._popup, firstElementChild, 5);
+			if (suggestionCount) {
+				let separator = this._popup.ownerDocument.createElement('menuseparator');
+				this._popup.insertBefore(separator, firstElementChild);
+			}
+		}
+		
 		this._popup.openPopupAtScreen(x, y, true);
 	}
 
+	_getSpellChecker() {
+		let spellChecker = new InlineSpellChecker();
+		let editingSession = this._iframeWindow
+			.getInterface(Ci.nsIWebNavigation)
+			.QueryInterface(Ci.nsIInterfaceRequestor)
+			.getInterface(Ci.nsIEditingSession);
+		spellChecker.init(editingSession.getEditorForWindow(this._iframeWindow));
+		return spellChecker;
+	}
+
 	async _ensureNoteCreated() {
 		if (!this._item.id) {
 			return this._item.saveTx();
@@ -861,6 +962,16 @@ class EditorInstance {
 			Zotero.crash(true);
 			throw e;
 		}
+		
+		// Reset spell checker as ProseMirror DOM modifications are
+		// often ignored otherwise
+		try {
+			let spellChecker = this._getSpellChecker();
+			spellChecker.toggleEnabled();
+			spellChecker.toggleEnabled();
+		} catch(e) {
+			Zotero.logError(e);
+		}
 	}
 
 	/**
diff --git a/chrome/content/zotero/xpcom/zotero.js b/chrome/content/zotero/xpcom/zotero.js
index 07ff35f031..1eb9b82076 100644
--- a/chrome/content/zotero/xpcom/zotero.js
+++ b/chrome/content/zotero/xpcom/zotero.js
@@ -725,6 +725,7 @@ Services.scriptloader.loadSubScript("resource://zotero/polyfill.js");
 			yield Zotero.Relations.init();
 			yield Zotero.Retractions.init();
 			yield Zotero.NoteBackups.init();
+			yield Zotero.Dictionaries.init();
 			
 			// Migrate fields from Extra that can be moved to item fields after a schema update
 			yield Zotero.Schema.migrateExtraFields();
diff --git a/components/zotero-service.js b/components/zotero-service.js
index f23545071f..b33060c94c 100644
--- a/components/zotero-service.js
+++ b/components/zotero-service.js
@@ -95,6 +95,7 @@ const xpcomFilesLocal = [
 	'data/searches',
 	'data/tags',
 	'db',
+	'dictionaries',
 	'duplicates',
 	'editorInstance',
 	'feedReader',
diff --git a/resource/config.js b/resource/config.js
index 0c3bb02272..4a480e3a40 100644
--- a/resource/config.js
+++ b/resource/config.js
@@ -28,6 +28,7 @@ var ZOTERO_CONFIG = {
 	CREDITS_URL: 'https://www.zotero.org/support/credits_and_acknowledgments',
 	LICENSING_URL: 'https://www.zotero.org/support/licensing',
 	GET_INVOLVED_URL: 'https://www.zotero.org/getinvolved',
+	DICTIONARIES_URL: 'https://download.zotero.org/dictionaries/',
 };
 
 if (typeof process === 'object' && process + '' === '[object process]'){