diff --git a/tildes/static/js/behaviors/comment-reply-button.js b/tildes/static/js/behaviors/comment-reply-button.js
index 0f94ad4..e0af3fb 100644
--- a/tildes/static/js/behaviors/comment-reply-button.js
+++ b/tildes/static/js/behaviors/comment-reply-button.js
@@ -79,6 +79,23 @@ $.onmount("[data-js-comment-reply-button]", function() {
             }
         }
 
+        var parentCommentTimestamp = new Date(
+            $parentComment
+                .find(".comment-posted-time")
+                .first()
+                .attr("datetime")
+        );
+
+        // add a warning if the comment being replied to is over a week old
+        if (Date.now() - parentCommentTimestamp > 1000 * 3600 * 24 * 7) {
+            var warningDiv = document.createElement("div");
+            warningDiv.classList.add("toast", "toast-minor", "toast-warning");
+            warningDiv.innerHTML =
+                "<h2>The comment you're replying to is over a week old.</h2>" +
+                "<p>(Replying to old comments is fine, just making sure it's intentional)</p>";
+            clone.querySelector("form").prepend(warningDiv);
+        }
+
         // update Intercooler so it knows about this new form
         Intercooler.processNodes(clone);
 
diff --git a/tildes/tildes/templates/topic.jinja2 b/tildes/tildes/templates/topic.jinja2
index 73c4697..985a8c6 100644
--- a/tildes/tildes/templates/topic.jinja2
+++ b/tildes/tildes/templates/topic.jinja2
@@ -213,6 +213,12 @@
       data-js-prevent-double-submit
       data-js-confirm-leave-page-unsaved
     >
+      {% if old_topic_warning %}
+      <div class="toast toast-minor toast-warning">
+        <h2>This topic is over a week old.</h2>
+        <p>(Replying to old topics is fine, just making sure it's intentional)</p>
+      </div>
+      {% endif %}
       <input type="hidden" name="csrf_token" value="{{ get_csrf_token() }}">
 
       {{ markdown_textarea() }}
diff --git a/tildes/tildes/views/topic.py b/tildes/tildes/views/topic.py
index 7afe9f2..d41c6fe 100644
--- a/tildes/tildes/views/topic.py
+++ b/tildes/tildes/views/topic.py
@@ -6,6 +6,7 @@
 from collections import namedtuple
 from typing import Any, Optional, Union
 
+from datetime import timedelta
 from marshmallow import missing, ValidationError
 from marshmallow.fields import Boolean, String
 from pyramid.httpexceptions import HTTPFound
@@ -374,6 +375,8 @@ def get_topic(request: Request, comment_order: CommentTreeSortOption) -> dict:
             tree.uncollapse_new_comments(topic.last_visit_time)
             tree.finalize_collapsing_maximized()
 
+    old_topic_warning = topic.age > timedelta(days=7)
+
     return {
         "topic": topic,
         "log": log,
@@ -381,6 +384,7 @@ def get_topic(request: Request, comment_order: CommentTreeSortOption) -> dict:
         "comment_order": comment_order,
         "comment_order_options": CommentTreeSortOption,
         "comment_label_options": CommentLabelOption,
+        "old_topic_warning": old_topic_warning,
     }