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
@@ -196,6 +196,8 @@
     'ArcanistInvalidModifiersXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistInvalidModifiersXHPASTLinterRuleTestCase.php',
     'ArcanistInvalidOctalNumericScalarXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistInvalidOctalNumericScalarXHPASTLinterRule.php',
     'ArcanistInvalidOctalNumericScalarXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistInvalidOctalNumericScalarXHPASTLinterRuleTestCase.php',
+    'ArcanistIsAShouldBeInstanceOfXHPASTLinterRule' => 'lint/linter/xhpast/rules/ArcanistIsAShouldBeInstanceOfXHPASTLinterRule.php',
+    'ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase' => 'lint/linter/xhpast/rules/__tests__/ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase.php',
     'ArcanistJSHintLinter' => 'lint/linter/ArcanistJSHintLinter.php',
     'ArcanistJSHintLinterTestCase' => 'lint/linter/__tests__/ArcanistJSHintLinterTestCase.php',
     'ArcanistJSONLintLinter' => 'lint/linter/ArcanistJSONLintLinter.php',
@@ -608,6 +610,8 @@
     'ArcanistInvalidModifiersXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistInvalidOctalNumericScalarXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
     'ArcanistInvalidOctalNumericScalarXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
+    'ArcanistIsAShouldBeInstanceOfXHPASTLinterRule' => 'ArcanistXHPASTLinterRule',
+    'ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase' => 'ArcanistXHPASTLinterRuleTestCase',
     'ArcanistJSHintLinter' => 'ArcanistExternalLinter',
     'ArcanistJSHintLinterTestCase' => 'ArcanistExternalLinterTestCase',
     'ArcanistJSONLintLinter' => 'ArcanistExternalLinter',
diff --git a/src/lint/linter/xhpast/rules/ArcanistIsAShouldBeInstanceOfXHPASTLinterRule.php b/src/lint/linter/xhpast/rules/ArcanistIsAShouldBeInstanceOfXHPASTLinterRule.php
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/ArcanistIsAShouldBeInstanceOfXHPASTLinterRule.php
@@ -0,0 +1,69 @@
+<?php
+
+final class ArcanistIsAShouldBeInstanceOfXHPASTLinterRule
+  extends ArcanistXHPASTLinterRule {
+
+  const ID = 111;
+
+  public function getLintName() {
+    return pht('`%s` Should Be `%s`', 'is_a', 'instanceof');
+  }
+
+  public function getLintSeverity() {
+    return ArcanistLintSeverity::SEVERITY_ADVICE;
+  }
+
+  public function process(XHPASTNode $root) {
+    $calls = $this->getFunctionCalls($root, array('is_a'));
+
+    foreach ($calls as $call) {
+      $parameters = $call->getChildOfType(1, 'n_CALL_PARAMETER_LIST');
+
+      if (count($parameters->getChildren()) > 2) {
+        // If the `$allow_string` parameter is `true` then the `instanceof`
+        // operator cannot be used. Evaluating whether an expression is truthy
+        // or falsely is hard, and so we only check that the `$allow_string`
+        // parameter is either absent or literally `false`.
+        $allow_string = $parameters->getChildByIndex(2);
+
+        if (strtolower($allow_string->getConcreteString()) != 'false') {
+          continue;
+        }
+      }
+
+      $object = $parameters->getChildByIndex(0);
+      $class  = $parameters->getChildByIndex(1);
+
+      switch ($class->getTypeName()) {
+        case 'n_STRING_SCALAR':
+          $replacement = stripslashes(
+            substr($class->getConcreteString(), 1, -1));
+          break;
+
+        case 'n_VARIABLE':
+          $replacement = $class->getConcreteString();
+          break;
+
+        default:
+          $replacement = null;
+          break;
+      }
+
+      $this->raiseLintAtNode(
+        $call,
+        pht(
+          'Use `%s` instead of `%s`. The former is a language '.
+          'construct whereas the latter is a function call, which '.
+          'has additional overhead.',
+          'instanceof',
+          'is_a'),
+        ($replacement === null)
+          ? null
+          : sprintf(
+            '%s instanceof %s',
+            $object->getConcreteString(),
+            $replacement));
+    }
+  }
+
+}
diff --git a/src/lint/linter/xhpast/rules/__tests__/ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase.php b/src/lint/linter/xhpast/rules/__tests__/ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase.php
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase.php
@@ -0,0 +1,11 @@
+<?php
+
+final class ArcanistIsAShouldBeInstanceOfXHPASTLinterRuleTestCase
+  extends ArcanistXHPASTLinterRuleTestCase {
+
+  public function testLinter() {
+    $this->executeTestsInDirectory(
+      dirname(__FILE__).'/is_a-should-be-instanceof/');
+  }
+
+}
diff --git a/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/allow_string.lint-test b/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/allow_string.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/allow_string.lint-test
@@ -0,0 +1,3 @@
+<?php
+is_a('SubClass', 'BaseClass', true);
+~~~~~~~~~~
diff --git a/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/is_a.lint-test b/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/is_a.lint-test
new file mode 100644
--- /dev/null
+++ b/src/lint/linter/xhpast/rules/__tests__/is_a-should-be-instanceof/is_a.lint-test
@@ -0,0 +1,16 @@
+<?php
+is_a($x, $y);
+is_a($x, 'SomeClass');
+is_a($x, '\\SomeClass');
+is_a($x, '\\'.'Some'.'Class');
+~~~~~~~~~~
+advice:2:1
+advice:3:1
+advice:4:1
+advice:5:1
+~~~~~~~~~~
+<?php
+$x instanceof $y;
+$x instanceof SomeClass;
+$x instanceof \SomeClass;
+is_a($x, '\\'.'Some'.'Class');