diff --git a/changelog.d/6375.bugfix b/changelog.d/6375.bugfix
new file mode 100644
index 000000000000..d023b49181cb
--- /dev/null
+++ b/changelog.d/6375.bugfix
@@ -0,0 +1 @@
+Fix `to_device` stream ID getting reset every time Synapse restarts, which had the potential to cause unable to decrypt errors.
\ No newline at end of file
diff --git a/synapse/storage/data_stores/main/deviceinbox.py b/synapse/storage/data_stores/main/deviceinbox.py
index 96cd0fb77ade..f04aad074339 100644
--- a/synapse/storage/data_stores/main/deviceinbox.py
+++ b/synapse/storage/data_stores/main/deviceinbox.py
@@ -358,21 +358,8 @@ def add_messages_txn(txn, now_ms, stream_id):
     def _add_messages_to_local_device_inbox_txn(
         self, txn, stream_id, messages_by_user_then_device
     ):
-        # Compatible method of performing an upsert
-        sql = "SELECT stream_id FROM device_max_stream_id"
-
-        txn.execute(sql)
-        rows = txn.fetchone()
-        if rows:
-            db_stream_id = rows[0]
-            if db_stream_id < stream_id:
-                # Insert the new stream_id
-                sql = "UPDATE device_max_stream_id SET stream_id = ?"
-        else:
-            # No rows, perform an insert
-            sql = "INSERT INTO device_max_stream_id (stream_id) VALUES (?)"
-
-        txn.execute(sql, (stream_id,))
+        sql = "UPDATE device_max_stream_id" " SET stream_id = ?" " WHERE stream_id < ?"
+        txn.execute(sql, (stream_id, stream_id))
 
         local_by_user_then_device = {}
         for user_id, messages_by_device in messages_by_user_then_device.items():
diff --git a/synapse/storage/data_stores/main/schema/delta/56/device_stream_id.sql b/synapse/storage/data_stores/main/schema/delta/56/device_stream_id.sql
new file mode 100644
index 000000000000..ae261a70a28b
--- /dev/null
+++ b/synapse/storage/data_stores/main/schema/delta/56/device_stream_id.sql
@@ -0,0 +1,5 @@
+INSERT INTO device_max_stream_id (stream_id)
+SELECT 0
+WHERE NOT EXISTS (
+    SELECT 1 FROM device_max_stream_id
+);
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