diff --git a/billing/v1/billing.proto b/billing/v1/billing.proto
index cde4e286..98ceb8cf 100644
--- a/billing/v1/billing.proto
+++ b/billing/v1/billing.proto
@@ -1376,7 +1376,8 @@ message RegisterAccounts {
   // For AWS, Register accountId under the specific payer account into the billing group
   bool enabled = 1;
 
-  // Optional. AccountId to be used for registration into the billing group.
+  // Optional. AccountIds to be used for registration into the billing group.
+  // Accepts multiple accountIds separated by a comma.
   string accountId = 2;
 }
 
diff --git a/openapiv2/apidocs.swagger.json b/openapiv2/apidocs.swagger.json
index 43c9ebab..e9310b16 100644
--- a/openapiv2/apidocs.swagger.json
+++ b/openapiv2/apidocs.swagger.json
@@ -44807,7 +44807,7 @@
         },
         "accountId": {
           "type": "string",
-          "description": "Optional. AccountId to be used for registration into the billing group."
+          "description": "Optional. AccountIds to be used for registration into the billing group.\nAccepts multiple accountIds separated by a comma."
         }
       },
       "title": "Register Accounts definition"