diff --git a/src/lexer/PhutilTypeLexer.php b/src/lexer/PhutilTypeLexer.php
--- a/src/lexer/PhutilTypeLexer.php
+++ b/src/lexer/PhutilTypeLexer.php
@@ -19,7 +19,7 @@
         array('map', 'map'),
         array('list', 'list'),
         array('int|float|bool|string|null|callable|wild|regex', 'k'),
-        array('[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*', 'k'),
+        array('\\\\?[a-zA-Z_\x7f-\xff]+(\\\\[a-zA-Z_\x7f-\xff]+)*', 'k'),
         array('\\(', '(', 'comment'),
       ),
       'comment' => array(
diff --git a/src/parser/__tests__/PhutilTypeSpecTestCase.php b/src/parser/__tests__/PhutilTypeSpecTestCase.php
--- a/src/parser/__tests__/PhutilTypeSpecTestCase.php
+++ b/src/parser/__tests__/PhutilTypeSpecTestCase.php
@@ -18,6 +18,10 @@
       'optional int? (minimum 300)',
       'list<wild>',
       'list<list<list<map<string, string>>>> (easy)',
+      '\\SomeClass',
+      '\\Namespace\\SomeClass',
+      '\\NamespaceA\\NamespaceB\\NamespaceC',
+      'NamespaceA\\NamespaceB\\NamespaceC',
     );
 
     $bad = array(
@@ -29,6 +33,10 @@
       '(derp)',
       'list<string (capital letters), string>',
       'int?|string',
+      '\\',
+      '\\\\',
+      '\\SomeClass\\',
+      'SomeClass\\',
     );
 
     $good = array_fill_keys($good, true);