diff --git a/API.md b/API.md
index 56e5fe20..e5b8571c 100644
--- a/API.md
+++ b/API.md
@@ -1576,6 +1576,7 @@ One file together with its metadata posted to the site.
     "commentCount":       <comment-count>,
     "noteCount":          <note-count>,
     "featureCount":       <feature-count>,
+    "relationCount":      <relation-count>,
     "lastFeatureTime":    <last-feature-time>,
     "favoritedBy":        <favorited-by>,
     "hasCustomThumbnail": <has-custom-thumbnail>,
@@ -1642,6 +1643,7 @@ One file together with its metadata posted to the site.
 - `<comment-count>`: how many comments are filed under that post
 - `<note-count>`: how many notes the post has
 - `<feature-count>`: how many times has the post been featured.
+- `<relation-count>`: how many posts are related to this post.
 - `<last-feature-time>`: the last time the post was featured, formatted as per
   RFC 3339.
 - `<favorited-by>`: list of users, serialized as [micro user resources](#micro-user).
diff --git a/server/szurubooru/func/posts.py b/server/szurubooru/func/posts.py
index 5d12cb4b..87b1cbee 100644
--- a/server/szurubooru/func/posts.py
+++ b/server/szurubooru/func/posts.py
@@ -109,6 +109,7 @@ def serialize_post(post, authenticated_user, options=None):
             'favoriteCount': lambda: post.favorite_count,
             'commentCount': lambda: post.comment_count,
             'noteCount': lambda: post.note_count,
+            'relationCount': lambda: post.relation_count,
             'featureCount': lambda: post.feature_count,
             'lastFeatureTime': lambda: post.last_feature_time,
             'favoritedBy': lambda: [
diff --git a/server/szurubooru/tests/func/test_posts.py b/server/szurubooru/tests/func/test_posts.py
index 899ee9e4..1fde0013 100644
--- a/server/szurubooru/tests/func/test_posts.py
+++ b/server/szurubooru/tests/func/test_posts.py
@@ -154,6 +154,7 @@ def test_serialize_post(
         'commentCount': 2,
         'noteCount': 0,
         'featureCount': 1,
+        'relationCount': 0,
         'lastFeatureTime': datetime.datetime(1999, 1, 1),
         'favoritedBy': ['fav1'],
         'hasCustomThumbnail': True,