diff --git a/lib/Fregot/Eval/Builtins.hs b/lib/Fregot/Eval/Builtins.hs
index e2af45fe..60ecd46b 100644
--- a/lib/Fregot/Eval/Builtins.hs
+++ b/lib/Fregot/Eval/Builtins.hs
@@ -220,6 +220,7 @@ defaultBuiltins = HMS.fromList
     , (NamedFunction (BuiltinName "indexof"),                   builtin_indexof)
     , (NamedFunction (BuiltinName "intersection"),              builtin_intersection)
     , (NamedFunction (BuiltinName "is_array"),                  builtin_is_array)
+    , (NamedFunction (BuiltinName "is_boolean"),                builtin_is_boolean)
     , (NamedFunction (BuiltinName "is_object"),                 builtin_is_object)
     , (NamedFunction (BuiltinName "is_string"),                 builtin_is_string)
     , (NamedFunction (QualifiedName "json" "unmarshal"),        builtin_json_unmarshal)
@@ -320,6 +321,12 @@ builtin_is_array = Builtin (In Out) $ pure $
         ArrayV _ -> return True
         _        -> return False
 
+builtin_is_boolean :: Monad m => Builtin m
+builtin_is_boolean = Builtin (In Out) $ pure $
+    \(Cons val Nil) -> case val of
+        BoolV _ -> return True
+        _       -> return False
+
 builtin_is_object :: Monad m => Builtin m
 builtin_is_object = Builtin (In Out) $ pure $
     \(Cons val Nil) -> case val of
diff --git a/tests/rego/builtins-06.rego b/tests/rego/builtins-06.rego
new file mode 100644
index 00000000..ebf0c549
--- /dev/null
+++ b/tests/rego/builtins-06.rego
@@ -0,0 +1,9 @@
+# 'is_' functions
+package fregot.tests.builtins_06
+
+test_is_boolean {
+  is_boolean(true)
+  is_boolean(false)
+  not is_boolean(1)
+  not is_boolean([true])
+}