diff --git a/analytical/templatetags/intercom.py b/analytical/templatetags/intercom.py
index f0f50d1..888a0ab 100644
--- a/analytical/templatetags/intercom.py
+++ b/analytical/templatetags/intercom.py
@@ -3,10 +3,15 @@ intercom.io template tags and filters.
 """
 
 from __future__ import absolute_import
+
+import hashlib
+import hmac
 import json
+import sys
 import time
 import re
 
+from django.conf import settings
 from django.template import Library, Node, TemplateSyntaxError
 
 from analytical.utils import disable_html, get_required_setting, \
@@ -24,6 +29,42 @@ TRACKING_CODE = """
 register = Library()
 
 
+def _timestamp(when):  # type: (datetime) -> float
+    """
+    Python 2 compatibility for `datetime.timestamp()`.
+    """
+    return (time.mktime(when.timetuple()) if sys.version_info < (3,) else
+            when.timestamp())
+
+
+def _hashable_bytes(data):  # type: (AnyStr) -> bytes
+    """
+    Coerce strings to hashable bytes.
+    """
+    if isinstance(data, bytes):
+        return data
+    elif isinstance(data, str):
+        return data.encode('ascii')  # Fail on anything non-ASCII.
+    else:
+        raise TypeError(data)
+
+
+def intercom_user_hash(data):  # type: (AnyStr) -> Optional[str]
+    """
+    Return a SHA-256 HMAC `user_hash` as expected by Intercom, if configured.
+
+    Return None if the `INTERCOM_HMAC_SECRET_KEY` setting is not configured.
+    """
+    if getattr(settings, 'INTERCOM_HMAC_SECRET_KEY', None):
+        return hmac.new(
+            key=_hashable_bytes(settings.INTERCOM_HMAC_SECRET_KEY),
+            msg=_hashable_bytes(data),
+            digestmod=hashlib.sha256,
+        ).hexdigest()
+    else:
+        return None
+
+
 @register.tag
 def intercom(parser, token):
     """
@@ -66,11 +107,22 @@ class IntercomNode(Node):
             if 'email' not in params and user.email:
                 params['email'] = user.email
 
-            params['created_at'] = int(time.mktime(
-                    user.date_joined.timetuple()))
+            params.setdefault('user_id', user.pk)
+
+            params['created_at'] = int(_timestamp(user.date_joined))
         else:
             params['created_at'] = None
 
+        # Generate a user_hash HMAC to verify the user's identity, if configured.
+        # (If both user_id and email are present, the user_id field takes precedence.)
+        # See:
+        # https://www.intercom.com/help/configure-intercom-for-your-product-or-site/staying-secure/enable-identity-verification-on-your-web-product
+        user_hash_data = params.get('user_id', params.get('email'))  # type: Optional[str]
+        if user_hash_data:
+            user_hash = intercom_user_hash(str(user_hash_data))  # type: Optional[str]
+            if user_hash is not None:
+                params.setdefault('user_hash', user_hash)
+
         return params
 
     def render(self, context):
diff --git a/analytical/tests/test_tag_intercom.py b/analytical/tests/test_tag_intercom.py
index bc24739..2085fcb 100644
--- a/analytical/tests/test_tag_intercom.py
+++ b/analytical/tests/test_tag_intercom.py
@@ -9,7 +9,7 @@ from django.http import HttpRequest
 from django.template import Context
 from django.test.utils import override_settings
 
-from analytical.templatetags.intercom import IntercomNode
+from analytical.templatetags.intercom import IntercomNode, intercom_user_hash, _timestamp
 from analytical.tests.utils import TagTestCase
 from analytical.utils import AnalyticalException
 
@@ -26,21 +26,21 @@ class IntercomTagTestCase(TagTestCase):
 
     def test_node(self):
         now = datetime.datetime(2014, 4, 9, 15, 15, 0)
-        rendered_tag = IntercomNode().render(Context({
-            'user': User(
-                username='test',
-                first_name='Firstname',
-                last_name='Lastname',
-                email="test@example.com",
-                date_joined=now),
-        }))
+        user = User.objects.create(
+            username='test',
+            first_name='Firstname',
+            last_name='Lastname',
+            email="test@example.com",
+            date_joined=now,
+        )
+        rendered_tag = IntercomNode().render(Context({'user': user}))
         # Because the json isn't predictably ordered, we can't just test the whole thing verbatim.
         self.assertEqual("""
 <script id="IntercomSettingsScriptTag">
-  window.intercomSettings = {"app_id": "abc123xyz", "created_at": 1397074500, "email": "test@example.com", "name": "Firstname Lastname"};
+  window.intercomSettings = {"app_id": "abc123xyz", "created_at": 1397074500, "email": "test@example.com", "name": "Firstname Lastname", "user_id": %(user_id)s};
 </script>
 <script>(function(){var w=window;var ic=w.Intercom;if(typeof ic==="function"){ic('reattach_activator');ic('update',intercomSettings);}else{var d=document;var i=function(){i.c(arguments)};i.q=[];i.c=function(args){i.q.push(args)};w.Intercom=i;function l(){var s=d.createElement('script');s.type='text/javascript';s.async=true;s.src='https://static.intercomcdn.com/intercom.v1.js';var x=d.getElementsByTagName('script')[0];x.parentNode.insertBefore(s,x);}if(w.attachEvent){w.attachEvent('onload',l);}else{w.addEventListener('load',l,false);}}})()</script>
-""", rendered_tag)  # noqa
+""" % {'user_id': user.pk}, rendered_tag)  # noqa
 
     @override_settings(INTERCOM_APP_ID=None)
     def test_no_account_number(self):
@@ -52,18 +52,21 @@ class IntercomTagTestCase(TagTestCase):
 
     def test_identify_name_email_and_created_at(self):
         now = datetime.datetime(2014, 4, 9, 15, 15, 0)
+        user = User.objects.create(
+            username='test',
+            first_name='Firstname',
+            last_name='Lastname',
+            email="test@example.com",
+            date_joined=now,
+        )
         r = IntercomNode().render(Context({
-            'user': User(
-                username='test',
-                first_name='Firstname',
-                last_name='Lastname',
-                email="test@example.com",
-                date_joined=now),
+            'user': user,
         }))
         self.assertTrue('window.intercomSettings = {'
                         '"app_id": "abc123xyz", "created_at": 1397074500, '
-                        '"email": "test@example.com", "name": "Firstname Lastname"'
-                        '};' in r)
+                        '"email": "test@example.com", "name": "Firstname Lastname", '
+                        '"user_id": %(user_id)s'
+                        '};' % {'user_id': user.pk} in r, msg=r)
 
     def test_custom(self):
         r = IntercomNode().render(Context({
@@ -100,6 +103,65 @@ class IntercomTagTestCase(TagTestCase):
         }))
         self.assertTrue('"email": "explicit"' in r, r)
 
+    @override_settings(INTERCOM_HMAC_SECRET_KEY='secret')
+    def test_user_hash__without_user_details(self):
+        """
+        No `user_hash` without `user_id` or `email`.
+        """
+        attrs = IntercomNode()._get_custom_attrs(Context())
+        self.assertEqual({
+            'created_at': None,
+        }, attrs)
+
+    @override_settings(INTERCOM_HMAC_SECRET_KEY='secret')
+    def test_user_hash__with_user(self):
+        """
+        'user_hash' of default `user_id`.
+        """
+        user = User.objects.create(
+            email='test@example.com',
+        )  # type: User
+        attrs = IntercomNode()._get_custom_attrs(Context({'user': user}))
+        self.assertEqual({
+            'created_at': int(_timestamp(user.date_joined)),
+            'email': 'test@example.com',
+            'name': '',
+            'user_hash': intercom_user_hash(str(user.pk)),
+            'user_id': user.pk,
+        }, attrs)
+
+    @override_settings(INTERCOM_HMAC_SECRET_KEY='secret')
+    def test_user_hash__with_explicit_user_id(self):
+        """
+        'user_hash' of context-provided `user_id`.
+        """
+        attrs = IntercomNode()._get_custom_attrs(Context({
+            'intercom_email': 'test@example.com',
+            'intercom_user_id': '5',
+        }))
+        self.assertEqual({
+            'created_at': None,
+            'email': 'test@example.com',
+            # HMAC for user_id:
+            'user_hash': 'd3123a7052b42272d9b520235008c248a5aff3221cc0c530b754702ad91ab102',
+            'user_id': '5',
+        }, attrs)
+
+    @override_settings(INTERCOM_HMAC_SECRET_KEY='secret')
+    def test_user_hash__with_explicit_email(self):
+        """
+        'user_hash' of context-provided `email`.
+        """
+        attrs = IntercomNode()._get_custom_attrs(Context({
+            'intercom_email': 'test@example.com',
+        }))
+        self.assertEqual({
+            'created_at': None,
+            'email': 'test@example.com',
+            # HMAC for email:
+            'user_hash': '49e43229ee99dca2565241719b8341b04e71dd4de0628f991b5bea30a526e153',
+        }, attrs)
+
     @override_settings(ANALYTICAL_INTERNAL_IPS=['1.1.1.1'])
     def test_render_internal_ip(self):
         req = HttpRequest()
diff --git a/docs/services/intercom.rst b/docs/services/intercom.rst
index 700fc1e..f458d28 100644
--- a/docs/services/intercom.rst
+++ b/docs/services/intercom.rst
@@ -120,6 +120,8 @@ Context variable       Description
 --------------------  -------------------------------------------
 ``intercom_email``      The visitor's email address.
 --------------------  -------------------------------------------
+``intercom_user_id``    The visitor's user id.
+--------------------  -------------------------------------------
 ``created_at``          The date the visitor created an account
 ====================  ===========================================
 
@@ -130,12 +132,29 @@ Context variable       Description
 Identifying authenticated users
 -------------------------------
 
-If you have not set the ``intercom_name`` or ``intercom_email`` variables
+If you have not set the ``intercom_name``, ``intercom_email``, or ``intercom_user_id`` variables
 explicitly, the username and email address of an authenticated user are
 passed to Intercom automatically.  See :ref:`identifying-visitors`.
 
 .. _intercom-internal-ips:
 
+
+Verifying identified users
+--------------------------
+
+Intercom supports HMAC authentication of users identified by user ID or email, in order to prevent impersonation.
+For more information, see `Enable identity verification on your web product`_ in the Intercom documentation.
+
+To enable this, configure your Intercom account's HMAC secret key::
+
+    INTERCOM_HMAC_SECRET_KEY = 'XXXXXXXXXXXXXXXXXXXXXXX'
+
+(You can find this secret key under the "Identity verification" section of your Intercom account settings page.)
+
+.. _`Enable identity verification on your web product`: https://www.intercom.com/help/configure-intercom-for-your-product-or-site/staying-secure/enable-identity-verification-on-your-web-product
+
+
+
 Internal IP addresses
 ---------------------