diff --git a/pillar/api/organizations/__init__.py b/pillar/api/organizations/__init__.py
index 72e9bec8..a95abc36 100644
--- a/pillar/api/organizations/__init__.py
+++ b/pillar/api/organizations/__init__.py
@@ -92,6 +92,21 @@ class OrgManager:
         unknown_users = set(emails) - {user['email'] for user in existing_user_docs}
         existing_users = {user['_id'] for user in existing_user_docs}
 
+        return self._assign_users(org_id, unknown_users, existing_users)
+
+    def assign_single_user(self, org_id: bson.ObjectId, *, user_id: bson.ObjectId) -> dict:
+        """Assigns a single, known user to the organization.
+
+        :returns: the new organization document.
+        """
+
+        self._log.info('Adding new member %s to organization %s', user_id, org_id)
+        return self._assign_users(org_id, set(), {user_id})
+
+    def _assign_users(self, org_id: bson.ObjectId,
+                      unknown_users: typing.Set[str],
+                      existing_users: typing.Set[bson.ObjectId]) -> dict:
+
         if self._log.isEnabledFor(logging.INFO):
             self._log.info('  - found users: %s', ', '.join(str(uid) for uid in existing_users))
             self._log.info('  - unknown users: %s', ', '.join(unknown_users))
diff --git a/pillar/api/organizations/patch.py b/pillar/api/organizations/patch.py
index 7cbce1a1..dff004fd 100644
--- a/pillar/api/organizations/patch.py
+++ b/pillar/api/organizations/patch.py
@@ -43,6 +43,27 @@ class OrganizationPatchHandler(patch_handler.AbstractPatchHandler):
         org_doc = current_app.org_manager.assign_users(org_id, emails)
         return jsonify(org_doc)
 
+    @authorization.require_login()
+    def patch_assign_user(self, org_id: bson.ObjectId, patch: dict):
+        """Assigns a single user by User ID to an organization.
+
+        The calling user must be admin of the organization.
+        """
+
+        self._assert_is_admin(org_id)
+
+        # Do some basic validation.
+        try:
+            user_id = patch['user_id']
+        except KeyError:
+            raise wz_exceptions.BadRequest('No key "user_id" in patch.')
+
+        user_oid = str2id(user_id)
+        log.info('User %s uses PATCH to add user %s to organization %s',
+                 current_user().user_id, user_oid, org_id)
+        org_doc = current_app.org_manager.assign_single_user(org_id, user_id=user_oid)
+        return jsonify(org_doc)
+
     @authorization.require_login()
     def patch_remove_user(self, org_id: bson.ObjectId, patch: dict):
         """Removes a user from an organization.
diff --git a/tests/test_api/test_organizations.py b/tests/test_api/test_organizations.py
index 9b7e9498..9268879a 100644
--- a/tests/test_api/test_organizations.py
+++ b/tests/test_api/test_organizations.py
@@ -193,6 +193,31 @@ class OrganizationPatchTest(AbstractPillarTest):
         self.assertEqual([str(member1_uid)], new_org_doc['members'])
         self.assertEqual(['member2@example.com'], new_org_doc['unknown_members'])
 
+    def test_assign_single_user(self):
+        self.enter_app_context()
+
+        admin_uid = self.create_user(24 * 'a', token='admin-token')
+        member1_uid = self.create_user(24 * 'b', email='member1@example.com')
+
+        om = self.app.org_manager
+        org_doc = om.create_new_org('Хакеры', admin_uid, 25)
+        org_id = org_doc['_id']
+
+        # Try the PATCH
+        resp = self.patch(f'/api/organizations/{org_id}',
+                          json={
+                              'op': 'assign-user',
+                              'user_id': str(member1_uid),
+                          },
+                          auth_token='admin-token')
+        new_org_doc = resp.get_json()
+
+        db = self.app.db('organizations')
+        db_org = db.find_one(org_id)
+
+        self.assertEqual([member1_uid], db_org['members'])
+        self.assertEqual([str(member1_uid)], new_org_doc['members'])
+
     def test_assign_users_access_denied(self):
         self.enter_app_context()