diff --git a/src/extensions/nip05/managers/nip05Manager.ts b/src/extensions/nip05/managers/nip05Manager.ts
index 3efe85b8..d83fef1c 100644
--- a/src/extensions/nip05/managers/nip05Manager.ts
+++ b/src/extensions/nip05/managers/nip05Manager.ts
@@ -55,10 +55,12 @@ function generateId(): string {
 }
 
 /**
- * Validate username format
- * - Lowercase alphanumeric and underscore only
+ * Validate username format per NIP-05 spec
+ * - Characters allowed: a-z, 0-9, hyphen (-), underscore (_), period (.)
  * - Must start with a letter
+ * - Must not end with a hyphen, underscore, or period
  * - Length within bounds
+ * - Special case: "_" alone is the root identifier (_@domain)
  */
 function validateUsername(username: string, config: Required<Nip05Config>): UsernameValidation {
   if (!username) {
@@ -67,6 +69,11 @@ function validateUsername(username: string, config: Required<Nip05Config>): User
 
   const normalized = username.toLowerCase().trim()
 
+  // Special case: root identifier "_" per NIP-05
+  if (normalized === '_') {
+    return { valid: true }
+  }
+
   if (normalized.length < config.min_username_length) {
     return { valid: false, error: `Username must be at least ${config.min_username_length} character(s)` }
   }
@@ -75,9 +82,10 @@ function validateUsername(username: string, config: Required<Nip05Config>): User
     return { valid: false, error: `Username must be at most ${config.max_username_length} characters` }
   }
 
-  // Only lowercase letters, numbers, and underscores
-  if (!/^[a-z][a-z0-9_]*$/.test(normalized)) {
-    return { valid: false, error: 'Username must start with a letter and contain only lowercase letters, numbers, and underscores' }
+  // NIP-05 spec: local-part MUST only use characters a-z0-9-_.
+  // Must start with a letter, must not end with separator
+  if (!/^[a-z][a-z0-9._-]*[a-z0-9]$/.test(normalized) && !/^[a-z]$/.test(normalized)) {
+    return { valid: false, error: 'Username must start with a letter, end with a letter or number, and contain only a-z, 0-9, hyphens, underscores, and periods' }
   }
 
   // Check reserved usernames