diff --git a/docs/companion.md b/docs/companion.md
index 29fbecc2b..586b37c61 100644
--- a/docs/companion.md
+++ b/docs/companion.md
@@ -463,6 +463,11 @@ which has only the secret, nothing else.
 
 :::
 
+##### `s3.endpoint` `COMPANION_AWS_ENDPOINT`
+
+Optional URL to a custom S3 (compatible) service. Otherwise uses the default
+from the AWS SDK.
+
 ##### `s3.bucket` `COMPANION_AWS_BUCKET`
 
 The name of the bucket to store uploaded files in.