diff --git a/src/api2/access/domain.rs b/src/api2/access/domain.rs
index 2b082939..a2fd746d 100644
--- a/src/api2/access/domain.rs
+++ b/src/api2/access/domain.rs
@@ -18,8 +18,7 @@ use crate::api2::types::*;
             description: "User configuration (without password).",
             properties: {
                 realm: {
-                    description: "Realm ID.",
-                    type: String,
+                    schema: REALM_ID_SCHEMA,
                 },
                 comment: {
                     schema: SINGLE_LINE_COMMENT_SCHEMA,
diff --git a/src/api2/types/mod.rs b/src/api2/types/mod.rs
index 2838a609..9d24f620 100644
--- a/src/api2/types/mod.rs
+++ b/src/api2/types/mod.rs
@@ -514,6 +514,12 @@ pub const BLOCKDEVICE_NAME_SCHEMA: Schema = StringSchema::new("Block device name
     .max_length(64)
     .schema();
 
+pub const REALM_ID_SCHEMA: Schema = StringSchema::new("Realm name.")
+    .format(&PROXMOX_SAFE_ID_FORMAT)
+    .min_length(2)
+    .max_length(32)
+    .schema();
+
 // Complex type definitions
 
 #[api(