diff --git a/apps/meteor/app/lib/server/functions/createRoom.ts b/apps/meteor/app/lib/server/functions/createRoom.ts
index 19f22ab6e7ac4..b936d30145069 100644
--- a/apps/meteor/app/lib/server/functions/createRoom.ts
+++ b/apps/meteor/app/lib/server/functions/createRoom.ts
@@ -143,6 +143,13 @@ export const createRoom = async <T extends RoomType>(
 		return member.username?.includes(':') && member.username?.includes('@');
 	});
 
+	// Prevent adding federated users to rooms that are not marked as federated explicitly
+	if (hasFederatedMembers && optionalExtraData.federated !== true) {
+		throw new Meteor.Error('error-federated-users-in-non-federated-rooms', 'Cannot add federated users to non-federated rooms', {
+			method: 'createRoom',
+		});
+	}
+
 	const extraData = {
 		...optionalExtraData,
 		...((hasFederatedMembers || optionalExtraData.federated) && {
diff --git a/apps/meteor/app/lib/server/methods/addUsersToRoom.ts b/apps/meteor/app/lib/server/methods/addUsersToRoom.ts
index 30830385b1dcd..c5fd718d6911f 100644
--- a/apps/meteor/app/lib/server/methods/addUsersToRoom.ts
+++ b/apps/meteor/app/lib/server/methods/addUsersToRoom.ts
@@ -1,7 +1,8 @@
 import { api } from '@rocket.chat/core-services';
 import type { IUser } from '@rocket.chat/core-typings';
-import { isRoomNativeFederated, isUserNativeFederated } from '@rocket.chat/core-typings';
+import { isRoomNativeFederated } from '@rocket.chat/core-typings';
 import type { ServerMethods } from '@rocket.chat/ddp-client';
+import { validateFederatedUsername } from '@rocket.chat/federation-matrix';
 import { Subscriptions, Users, Rooms } from '@rocket.chat/models';
 import { Match } from 'meteor/check';
 import { Meteor } from 'meteor/meteor';
@@ -88,15 +89,18 @@ export const addUsersToRoomMethod = async (userId: string, data: { rid: string;
 
 	await Promise.all(
 		data.users.map(async (username) => {
-			const newUser = await Users.findOneByUsernameIgnoringCase(sanitizeUsername(username));
-			if (!newUser) {
-				throw new Meteor.Error('error-user-not-found', 'User not found', {
+			const sanitizedUsername = sanitizeUsername(username);
+
+			// If it's a federated username format and the room is not federated, throw error immediately
+			if (validateFederatedUsername(sanitizedUsername) && !isRoomNativeFederated(room)) {
+				throw new Meteor.Error('error-federated-users-in-non-federated-rooms', 'Cannot add federated users to non-federated rooms', {
 					method: 'addUsersToRoom',
 				});
 			}
 
-			if (isUserNativeFederated(newUser) && !isRoomNativeFederated(room)) {
-				throw new Meteor.Error('error-federated-users-in-non-federated-rooms', 'Cannot add federated users to non-federated rooms', {
+			const newUser = await Users.findOneByUsernameIgnoringCase(sanitizedUsername);
+			if (!newUser) {
+				throw new Meteor.Error('error-user-not-found', 'User not found', {
 					method: 'addUsersToRoom',
 				});
 			}
diff --git a/apps/meteor/app/slashcommands-invite/server/server.ts b/apps/meteor/app/slashcommands-invite/server/server.ts
index 24ad0484fddcc..cc4e116226665 100644
--- a/apps/meteor/app/slashcommands-invite/server/server.ts
+++ b/apps/meteor/app/slashcommands-invite/server/server.ts
@@ -1,9 +1,11 @@
-import { api } from '@rocket.chat/core-services';
+import { api, FederationMatrix } from '@rocket.chat/core-services';
 import type { IUser, SlashCommandCallbackParams } from '@rocket.chat/core-typings';
-import { Subscriptions, Users } from '@rocket.chat/models';
+import { validateFederatedUsername } from '@rocket.chat/federation-matrix';
+import { Subscriptions, Users, Rooms } from '@rocket.chat/models';
 import { Meteor } from 'meteor/meteor';
 
 import { i18n } from '../../../server/lib/i18n';
+import { FederationActions } from '../../../server/services/room/hooks/BeforeFederationActions';
 import { addUsersToRoomMethod, sanitizeUsername } from '../../lib/server/methods/addUsersToRoom';
 import { settings } from '../../settings/server';
 import { slashCommands } from '../../utils/server/slashCommand';
@@ -15,13 +17,42 @@ import { slashCommands } from '../../utils/server/slashCommand';
 slashCommands.add({
 	command: 'invite',
 	callback: async ({ params, message, userId }: SlashCommandCallbackParams<'invite'>): Promise<void> => {
-		const usernames = params
+		let usernames = params
 			.split(/[\s,]/)
 			.map((username) => sanitizeUsername(username))
 			.filter((a) => a !== '');
 		if (usernames.length === 0) {
 			return;
 		}
+
+		// Get room information for federation check
+		const room = await Rooms.findOneById(message.rid);
+		if (!room) {
+			void api.broadcast('notify.ephemeralMessage', userId, message.rid, {
+				msg: i18n.t('error-invalid-room', { lng: settings.get('Language') || 'en' }),
+			});
+			return;
+		}
+
+		// Ensure federated users exist locally before looking them up
+		const federatedUsernames = usernames.filter((u) => validateFederatedUsername(u)) as string[];
+		if (federatedUsernames.length > 0) {
+			if (FederationActions.shouldPerformFederationAction(room)) {
+				await FederationMatrix.ensureFederatedUsersExistLocally(federatedUsernames);
+			} else {
+				void api.broadcast('notify.ephemeralMessage', userId, message.rid, {
+					msg: i18n.t('You_cannot_add_external_users_to_non_federated_room', { lng: settings.get('Language') || 'en' }),
+				});
+				// These federated users shouldn't be invited and we already broadcasted the error message
+				usernames = usernames.filter((username) => {
+					return !federatedUsernames.includes(username);
+				});
+				if (usernames.length === 0) {
+					return;
+				}
+			}
+		}
+
 		const users = await Users.findByUsernames(usernames).toArray();
 		if (users.length === 0) {
 			void api.broadcast('notify.ephemeralMessage', userId, message.rid, {
diff --git a/ee/packages/federation-matrix/src/index.ts b/ee/packages/federation-matrix/src/index.ts
index e7777aabbad28..b56729b4524ba 100644
--- a/ee/packages/federation-matrix/src/index.ts
+++ b/ee/packages/federation-matrix/src/index.ts
@@ -1,6 +1,6 @@
 import 'reflect-metadata';
 
-export { FederationMatrix } from './FederationMatrix';
+export { FederationMatrix, validateFederatedUsername } from './FederationMatrix';
 
 export { generateEd25519RandomSecretKey } from '@rocket.chat/federation-sdk';