diff --git a/tildes/tildes/routes.py b/tildes/tildes/routes.py
index 38b025e..ae2b7a2 100644
--- a/tildes/tildes/routes.py
+++ b/tildes/tildes/routes.py
@@ -183,6 +183,11 @@ def add_intercooler_routes(config: Configurator) -> None:
         '/user/{username}/default_listing_options',
         factory=user_by_username,
     )
+    add_ic_route(
+        'mark_notifications_read_before',
+        '/notifications/{last_notification_seen}/mark_read_before',
+        factory=LoggedInFactory,
+    )
 
 
 class LoggedInFactory:
diff --git a/tildes/tildes/templates/notifications_unread.jinja2 b/tildes/tildes/templates/notifications_unread.jinja2
index b580f1f..a29f248 100644
--- a/tildes/tildes/templates/notifications_unread.jinja2
+++ b/tildes/tildes/templates/notifications_unread.jinja2
@@ -5,7 +5,19 @@
 
 {% block title %}Unread notifications{% endblock %}
 
-{% block main_heading %}Unread notifications{% endblock %}
+{% block main_heading %}Unread notifications
+  {% if notifications and not request.user.auto_mark_notifications_read %}
+      <button
+        class="btn btn-link-minimal ml-2"
+        data-ic-put-to="{{ request.route_url(
+          'ic_mark_notifications_read_before',
+          last_notification_seen=notifications[0].created_time,
+        )}}",
+        data-ic-target="closest main"
+      >Mark all read</button>
+  {% endif %}
+
+{% endblock %}
 
 {% block content %}
 {% if notifications %}
diff --git a/tildes/tildes/views/api/web/notifications.py b/tildes/tildes/views/api/web/notifications.py
new file mode 100644
index 0000000..28c611d
--- /dev/null
+++ b/tildes/tildes/views/api/web/notifications.py
@@ -0,0 +1,73 @@
+"""Web API endpoints related to comment notifications."""
+
+from datetime import datetime
+
+from marshmallow.fields import DateTime
+from pyramid.request import Request
+from pyramid.response import Response
+from sqlalchemy.dialects.postgresql import insert
+from webargs.pyramidparser import use_kwargs
+from zope.sqlalchemy import mark_changed
+
+from tildes.lib.datetime import utc_now
+from tildes.models.comment import CommentNotification
+from tildes.models.topic import TopicVisit
+from tildes.views.decorators import ic_view_config
+
+
+@ic_view_config(
+    route_name='mark_notifications_read_before', request_method='PUT',
+)
+@use_kwargs(
+    {'last_notification_seen': DateTime(required=True)},
+    locations=('matchdict',),
+)
+def put_notifications_read_before(
+        request: Request,
+        last_notification_seen: datetime,
+) -> Response:
+    """Clear all user comment notifications before the latest displayed."""
+    notifications = (
+        request.query(CommentNotification)
+        .join_all_relationships()
+        .filter(
+            CommentNotification.user == request.user,
+            CommentNotification.is_unread == True,  # noqa
+            CommentNotification.created_time <= last_notification_seen,
+        )
+        .all()
+    )
+
+    # If the user has the "track comment visits" feature enabled, we want to
+    # increment the number of comments they've seen in the thread that the
+    # comment came from, so that they don't *both* get a notification as well
+    # as have the thread highlight with "(1 new)". This should only happen if
+    # their last visit was before the comment was posted, however.
+    # Below, this is implemented as a INSERT ... ON CONFLICT DO UPDATE so that
+    # it will insert a new topic visit with 1 comment if they didn't previously
+    # have one at all.
+    if request.user.track_comment_visits:
+        for notification in notifications:
+            statement = (
+                insert(TopicVisit.__table__)
+                .values(
+                    user_id=request.user.user_id,
+                    topic_id=notification.comment.topic_id,
+                    visit_time=utc_now(),
+                    num_comments=1,
+                )
+                .on_conflict_do_update(
+                    constraint=TopicVisit.__table__.primary_key,
+                    set_={'num_comments': TopicVisit.num_comments + 1},
+                    where=TopicVisit.visit_time <
+                    notification.comment.created_time,
+                )
+            )
+
+        request.db_session.execute(statement)
+        mark_changed(request.db_session)
+
+    for notification in notifications:
+        notification.is_unread = False
+
+    return Response('Your comment notifications have been cleared.')