diff --git a/core/db.py b/core/db.py
index 6cfadb5..ee12047 100644
--- a/core/db.py
+++ b/core/db.py
@@ -26,8 +26,8 @@ def find_activities(q: _Q) -> Iterable[_D]:
     return DB[CollectionName.ACTIVITIES.value].find(q)
 
 
-def update_one_activity(q: _Q, update: _Q) -> None:
-    DB[CollectionName.ACTIVITIES.value].update_one(q, update)
+def update_one_activity(q: _Q, update: _Q) -> bool:
+    return DB[CollectionName.ACTIVITIES.value].update_one(q, update).matched_count == 1
 
 
 def update_many_activities(q: _Q, update: _Q) -> None:
diff --git a/core/inbox.py b/core/inbox.py
index 9ebf78c..6c29383 100644
--- a/core/inbox.py
+++ b/core/inbox.py
@@ -174,6 +174,23 @@ def _like_process_inbox(like: ap.Like, new_meta: _NewMeta) -> None:
     )
 
 
+@process_inbox.register
+def _emoji_reaction_process_inbox(emoji_reaction: ap.EmojiReaction, new_meta: _NewMeta) -> None:
+    _logger.info(f"process_inbox activity={emoji_reaction!r}")
+    obj = emoji_reaction.get_object()
+    # Try to update an existing emoji reaction counter entry for the activity emoji
+    if not update_one_activity(
+        {**by_type(ap.ActivityType.CREATE), **by_object_id(obj.id),
+         "meta.emoji_reactions.emoji": emoji_reaction.content},
+        {"$inc": {"meta.emoji_reactions.$.count": 1}},
+    ):
+        # Bootstrap the current emoji counter
+        update_one_activity(
+            {**by_type(ap.ActivityType.CREATE), **by_object_id(obj.id)},
+            {"$push": {"meta.emoji_reactions": {"emoji": emoji_reaction.content, "count": 1}}},
+        )
+
+
 @process_inbox.register
 def _follow_process_inbox(activity: ap.Follow, new_meta: _NewMeta) -> None:
     _logger.info(f"process_inbox activity={activity!r}")
diff --git a/core/meta.py b/core/meta.py
index 6a0fc52..1190cfe 100644
--- a/core/meta.py
+++ b/core/meta.py
@@ -70,6 +70,8 @@ class MetaKey(Enum):
     COUNT_BOOST = "count_boost"
     COUNT_REPLY = "count_reply"
 
+    EMOJI_REACTIONS = "emoji_reactions"
+
 
 def _meta(mk: MetaKey) -> str:
     return f"meta.{mk.value}"