diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -226,6 +226,8 @@
     'ArcanistModifierOrderingXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistModifierOrderingXHPASTLinterRuleTestCase.php',
     'ArcanistNamingConventionsXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistNamingConventionsXHPASTLinterRule.php',
     'ArcanistNamingConventionsXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistNamingConventionsXHPASTLinterRuleTestCase.php',
+    'ArcanistNestedNamespacesXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistNestedNamespacesXHPASTLinterRule.php',
+    'ArcanistNestedNamespacesXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistNestedNamespacesXHPASTLinterRuleTestCase.php',
     'ArcanistNewlineAfterOpenTagXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistNewlineAfterOpenTagXHPASTLinterRule.php',
     'ArcanistNewlineAfterOpenTagXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistNewlineAfterOpenTagXHPASTLinterRuleTestCase.php',
     'ArcanistNoEffectException' => 'exception/usage/ArcanistNoEffectException.php',
@@ -598,6 +600,8 @@
     'ArcanistModifierOrderingXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistNamingConventionsXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistNamingConventionsXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
+    'ArcanistNestedNamespacesXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
+    'ArcanistNestedNamespacesXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistNewlineAfterOpenTagXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistNewlineAfterOpenTagXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistNoEffectException' => 'ArcanistUsageException',
diff --git a/src/lint/linter/xhpast/rules/ArcanistNestedNamespacesXHPASTLinterRule.php b/src/lint/linter/xhpast/rules/ArcanistNestedNamespacesXHPASTLinterRule.php
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/ArcanistNestedNamespacesXHPASTLinterRule.php
@@ -0,0 +1,29 @@
+<?php
+
+final class ArcanistNestedNamespacesXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 90;
+
+  public function getLintName() {
+    return pht('Nested `%s` Statements', 'namespace');
+  }
+
+  public function process(XHPASTNode $root) {
+    $namespaces = $root->selectDescendantsOfType('n_NAMESPACE');
+
+    foreach ($namespaces as $namespace) {
+      $nested_namespaces = $namespace->selectDescendantsOfType('n_NAMESPACE');
+
+      foreach ($nested_namespaces as $nested_namespace) {
+        $this->raiseLintAtNode(
+          $nested_namespace,
+          pht(
+            '`%s` declarations cannot be nested. '.
+            'This construct will cause a PHP fatal error.',
+            'namespace'));
+      }
+    }
+  }
+
+}
diff --git a/src/lint/linter/xhpast/rules/__tests__/ArcanistNestedNamespacesXHPASTLinterRuleTestCase.php b/src/lint/linter/xhpast/rules/__tests__/ArcanistNestedNamespacesXHPASTLinterRuleTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/ArcanistNestedNamespacesXHPASTLinterRuleTestCase.php
@@ -0,0 +1,10 @@
+<?php
+
+final class ArcanistNestedNamespacesXHPASTLinterRuleTestCase
+  extends ArcanistXHPASTLinterRuleTestCase {
+
+  public function testLinter() {
+    $this->executeTestsInDirectory(dirname(__FILE__).'/nested-namespaces/');
+  }
+
+}
diff --git a/src/lint/linter/xhpast/rules/__tests__/nested-namespaces/nested.lint-test b/src/lint/linter/xhpast/rules/__tests__/nested-namespaces/nested.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/nested-namespaces/nested.lint-test
@@ -0,0 +1,9 @@
+<?php
+namespace X {
+  namespace Y {
+    namespace Z {}
+  }
+}
+~~~~~~~~~~
+error:3:3
+error:4:5