diff --git a/.changeset/fix-slash-command-multiline-truncation.md b/.changeset/fix-slash-command-multiline-truncation.md
new file mode 100644
index 0000000000000..e5f47ad0b5805
--- /dev/null
+++ b/.changeset/fix-slash-command-multiline-truncation.md
@@ -0,0 +1,5 @@
+---
+'@rocket.chat/meteor': patch
+---
+
+Fixes slash command params being silently truncated when the message contains a newline (Shift+Enter). The client-side regex in `processSlashCommand.ts` now uses the dotall (`s`) flag so multi-line params are passed to the command handler in full, matching the existing behaviour of the server-side parser.
diff --git a/apps/meteor/client/lib/chats/flows/processSlashCommand.spec.ts b/apps/meteor/client/lib/chats/flows/processSlashCommand.spec.ts
new file mode 100644
index 0000000000000..250ca4692c46a
--- /dev/null
+++ b/apps/meteor/client/lib/chats/flows/processSlashCommand.spec.ts
@@ -0,0 +1,96 @@
+import { processSlashCommand } from './processSlashCommand';
+import { createFakeMessage } from '../../../../tests/mocks/data';
+import type { ChatAPI } from '../ChatAPI';
+import { slashCommands } from '../../../../app/utils/client';
+import { sdk } from '../../../../app/utils/client/lib/SDKClient';
+
+jest.mock('../../../../app/authorization/client', () => ({
+	hasAtLeastOnePermission: jest.fn(() => true),
+}));
+
+jest.mock('../../../../app/utils/client', () => ({
+	slashCommands: {
+		commands: {},
+	},
+}));
+
+jest.mock('../../../../app/utils/client/lib/SDKClient', () => ({
+	sdk: {
+		rest: {
+			post: jest.fn().mockResolvedValue({ result: {} }),
+		},
+	},
+}));
+
+jest.mock('../../../../app/utils/lib/i18n', () => ({
+	t: jest.fn((key) => key),
+}));
+
+jest.mock('../../settings', () => ({
+	settings: {
+		peek: jest.fn(() => true),
+	},
+}));
+
+const mockChat = {
+	uid: 'user-123',
+	composer: {
+		clear: jest.fn(),
+	},
+	ActionManager: {
+		generateTriggerId: jest.fn(() => 'trigger-123'),
+		notifyBusy: jest.fn(),
+		notifyIdle: jest.fn(),
+	},
+	data: {
+		pushEphemeralMessage: jest.fn(),
+	},
+} as unknown as ChatAPI;
+
+const message = createFakeMessage({
+	_id: 'message-id',
+	rid: 'room-id',
+});
+
+describe('processSlashCommand', () => {
+	beforeEach(() => {
+		jest.clearAllMocks();
+		slashCommands.commands['test'] = {
+			command: 'test',
+			clientOnly: false,
+		} as any;
+	});
+
+	it('should parse single-line parameters correctly', async () => {
+		await processSlashCommand(mockChat, { ...message, msg: '/test param1 param2' });
+
+		expect(sdk.rest.post).toHaveBeenCalledWith('/v1/commands.run', {
+			command: 'test',
+			params: ' param1 param2',
+			roomId: message.rid,
+			triggerId: 'trigger-123',
+		});
+	});
+
+	it('should parse parameters spanning multiple lines correctly (dotall support)', async () => {
+		await processSlashCommand(mockChat, { ...message, msg: '/test param1\nparam2' });
+
+		expect(sdk.rest.post).toHaveBeenCalledWith('/v1/commands.run', {
+			command: 'test',
+			params: ' param1\nparam2',
+			roomId: message.rid,
+			triggerId: 'trigger-123',
+		});
+	});
+
+	it('should parse quoted parameters spanning multiple lines correctly', async () => {
+		await processSlashCommand(mockChat, { ...message, msg: '/test "quoted\nparam"' });
+
+		expect(sdk.rest.post).toHaveBeenCalledWith('/v1/commands.run', {
+			command: 'test',
+			params: ' "quoted\nparam"',
+			roomId: message.rid,
+			triggerId: 'trigger-123',
+		});
+	});
+});
diff --git a/apps/meteor/client/lib/chats/flows/processSlashCommand.ts b/apps/meteor/client/lib/chats/flows/processSlashCommand.ts
index bfd6a87ba3736..331604213c4ee 100644
--- a/apps/meteor/client/lib/chats/flows/processSlashCommand.ts
+++ b/apps/meteor/client/lib/chats/flows/processSlashCommand.ts
@@ -10,7 +10,7 @@ import { settings } from '../../settings';
 import type { ChatAPI } from '../ChatAPI';
 
 const parse = (msg: string): { command: string; params: string } | { command: SlashCommand; params: string } | undefined => {
-	const match = msg.match(/^\/([^\s]+)(.*)/);
+	const match = msg.match(/^\/([^\s]+)(.*)/s);
 
 	if (!match) {
 		return undefined;