diff --git a/test/api/v3/integration/chat/POST-chat.flag.test.js b/test/api/v3/integration/chat/POST-chat.flag.test.js
index 187a86614b..9c2d5849a9 100644
--- a/test/api/v3/integration/chat/POST-chat.flag.test.js
+++ b/test/api/v3/integration/chat/POST-chat.flag.test.js
@@ -8,7 +8,7 @@ describe('POST /chat/:chatId/flag', () => {
   let user, admin, anotherUser, group;
   const TEST_MESSAGE = 'Test Message';
 
-  before(async () => {
+  beforeEach(async () => {
     user = await generateUser({balance: 1});
     admin = await generateUser({balance: 1, 'contributor.admin': true});
     anotherUser = await generateUser();
@@ -83,6 +83,41 @@ describe('POST /chat/:chatId/flag', () => {
     expect(messageToCheck.flagCount).to.equal(5);
   });
 
+  it('allows admin to flag a message in a private group', async () => {
+    let privateGroup = await user.post('/groups', {
+      name: 'Test party',
+      type: 'party',
+      privacy: 'private',
+    });
+    let { message } = await user.post(`/groups/${privateGroup._id}/chat`, {message: TEST_MESSAGE});
+
+    let flagResult = await admin.post(`/groups/${privateGroup._id}/chat/${message.id}/flag`);
+
+    expect(flagResult.flags[admin._id]).to.equal(true);
+    expect(flagResult.flagCount).to.equal(5);
+
+    let groupWithFlags = await user.get(`/groups/${privateGroup._id}`);
+    let messageToCheck = find(groupWithFlags.chat, {id: message.id});
+
+    expect(messageToCheck).to.not.exist;
+  });
+
+  it('does not allow non member to flag message in private group', async () => {
+    let privateGroup = await user.post('/groups', {
+      name: 'Test party',
+      type: 'party',
+      privacy: 'private',
+    });
+    let { message } = await user.post(`/groups/${privateGroup._id}/chat`, {message: TEST_MESSAGE});
+
+    await expect(anotherUser.post(`/groups/${privateGroup._id}/chat/${message.id}/flag`))
+      .to.eventually.be.rejected.and.eql({
+        code: 404,
+        error: 'NotFound',
+        message: t('groupNotFound'),
+      });
+  });
+
   it('Returns an error when user tries to flag a message that is already flagged', async () => {
     let { message } = await anotherUser.post(`/groups/${group._id}/chat`, {message: TEST_MESSAGE});
 
diff --git a/website/server/controllers/api-v3/chat.js b/website/server/controllers/api-v3/chat.js
index 51838994bf..d1179b0ee2 100644
--- a/website/server/controllers/api-v3/chat.js
+++ b/website/server/controllers/api-v3/chat.js
@@ -159,14 +159,27 @@ api.likeChat = {
 
 /**
  * @api {post} /api/v3/groups/:groupId/chat/:chatId/flag Flag a group chat message
- * @apiVersion 3.0.0
+ * @apiDescription A message will be hidden from chat if two or more users flag a message. It will be hidden immediately if a moderator flags the message. An email is sent to the moderators about every flagged message.
  * @apiName FlagChat
  * @apiGroup Chat
  *
- * @apiParam {UUID} groupId The group _id ('party' for the user party and 'habitrpg' for tavern are accepted)
+ * @apiParam {UUID} groupId The group id ('party' for the user party and 'habitrpg' for tavern are accepted)
  * @apiParam {UUID} chatId The chat message id
  *
  * @apiSuccess {Object} data The flagged chat message
+ * @apiSuccess {UUID} data.id The id of the message
+ * @apiSuccess {String} data.text The text of the message
+ * @apiSuccess {Number} data.timestamp The timestamp of the message in milliseconds
+ * @apiSuccess {Object} data.likes The likes of the message
+ * @apiSuccess {Object} data.flags The flags of the message
+ * @apiSuccess {Number} data.flagCount The number of flags the message has
+ * @apiSuccess {UUID} data.uuid The user id of the author of the message
+ * @apiSuccess {String} data.user The username of the author of the message
+ *
+ * @apiError GroupNotFound Group could not be found or you don't have access
+ * @apiError ChatNotFound Chat message with specified id could not be found
+ * @apiError FlagOwnMessage Chat messages cannot be flagged by the author of the message
+ * @apiError AlreadyFlagged Chat messages cannot be flagged more than once by a user
  */
 api.flagChat = {
   method: 'POST',
@@ -182,7 +195,11 @@ api.flagChat = {
     let validationErrors = req.validationErrors();
     if (validationErrors) throw validationErrors;
 
-    let group = await Group.getGroup({user, groupId});
+    let group = await Group.getGroup({
+      user,
+      groupId,
+      optionalMembership: user.contributor.admin,
+    });
     if (!group) throw new NotFound(res.t('groupNotFound'));
     let message = _.find(group.chat, {id: req.params.chatId});