diff --git a/backend/cl-json.lisp b/backend/cl-json.lisp
index b7fbcef..611deae 100644
--- a/backend/cl-json.lisp
+++ b/backend/cl-json.lisp
@@ -76,8 +76,7 @@ such tokens is :SYMBOL)."
   `(let ((json::+json-lisp-symbol-tokens+
            '(("true" . t)
              ("false" . nil)
-             ("null" . :null)
-             ("undefined" . :undefined)))
+             ("null" . :null)))
          (json:*object-scope-variables* '(json:*internal-decoder* *json-object-accumulator* *json-last-object-key*))
          (json:*beginning-of-object-handler* #'json-object-init)
          (json:*object-key-handler* #'json-object-add-key)
diff --git a/functions.lisp b/functions.lisp
index 107026b..5d0339e 100644
--- a/functions.lisp
+++ b/functions.lisp
@@ -229,7 +229,6 @@ OBJECT can be JSON array or object, which in Lisp translates to
 (defgeneric jcopy (object)
   (:method ((object real)) object)
   (:method ((object (eql :null))) object)
-  (:method ((object (eql :undefined))) object)
   (:method ((object (eql t))) object)
   (:method ((object null)) object)
   (:method ((object string)) object)
@@ -268,12 +267,12 @@ If the OBJECT is not a JSON array/object, throws `non-indexable'."))
     (declare (ignore object))
     t)
   (:method ((object symbol))
-    (not (member object (list nil :null :undefined))))
+    (not (member object (list nil :null))))
   (:documentation "Test OBJECT for truthiness in JSON terms.
 
-Recognize all the values true, except for null, undefined, and
-false. This is to make the transition from JSON to Lisp (3+ false
-values -> 1 false value) smoother.
+Recognize all the values true, except for null and false. This is to
+make the transition from JSON to Lisp (2 false values -> 1 false
+value) smoother.
 
 Unlike JavaScript, empty strings and zero are not false (because this
 behavior is confusing)."))
diff --git a/macros.lisp b/macros.lisp
index c06a93d..12d9713 100644
--- a/macros.lisp
+++ b/macros.lisp
@@ -120,7 +120,7 @@ See more examples in njson tests."
         (bindings (list)))
     (labels ((parse-pattern (pattern &optional (current-path (list)))
                (etypecase pattern
-                 ((or (member :true :false :null :undefined) string real)
+                 ((or (member :true :false :null) string real)
                   (push (cons pattern (copy-list current-path))
                         bindings))
                  ((cons symbol *)
@@ -152,7 +152,7 @@ See more examples in njson tests."
                                                         (cons symbol (or (cons symbol null)
                                                                          null))))
                              if (typep binding '(or array real null
-                                                 (member :true :false :null :undefined)))
+                                                 (member :true :false :null)))
                                collect `(,(gensym) (check-value ,binding (vector ,@key) ,form-sym))
                              else if (and (symbolp binding)
                                           (uiop:emptyp key))
diff --git a/njson.lisp b/njson.lisp
index fb244ea..32a68b8 100644
--- a/njson.lisp
+++ b/njson.lisp
@@ -39,7 +39,6 @@ FROM can be a string, stream, pathname, or byte array.
 Distinguishes between null/false and arrays/objects.
 Decodes:
 - null as :NULL,
-- undefined as :UNDEFINED,
 - false as nil,
 - true as t,
 - arrays as vectors,
@@ -88,5 +87,4 @@ TO can be:
 Distinguishes between null and false.
 Encodes:
 - :NULL as null,
-- :UNDEFINED as undefined,
 - nil as false."))
diff --git a/tests/tests.lisp b/tests/tests.lisp
index 8601193..17533e7 100644
--- a/tests/tests.lisp
+++ b/tests/tests.lisp
@@ -66,7 +66,6 @@
     (assert-eql 8 (jcopy 8))
     (assert-float-equal 1.3 (jcopy 1.3))
     (assert-eq :null (jcopy :null))
-    (assert-eq :undefined (jcopy :undefined))
     (assert-error 'error (jcopy :whatever))
     (assert-eq t (jcopy t))
     (assert-false (jcopy nil))
@@ -98,7 +97,6 @@
     (assert-error 'non-indexable (jget 20 nil))
     (assert-error 'non-indexable (jget 20 t))
     (assert-error 'non-indexable (jget 20 :null))
-    (assert-error 'non-indexable (jget 20 :undefined))
     (assert-error 'non-indexable (jget 20 200))
     (assert-error 'non-indexable (jget 20 200.3))
     (assert-error 'non-indexable (jget 20 "foo"))
@@ -129,7 +127,6 @@
   (assert-equalp #(t) (ensure-array t))
   (assert-equalp #(nil) (ensure-array nil))
   (assert-equalp #(:null) (ensure-array :null))
-  (assert-equalp #(:undefined) (ensure-array :undefined))
   (assert-equalp #(8) (ensure-array 8))
   (assert-equalp #(1.3) (ensure-array 1.3))
   (assert-equalp #(1.3 "foo") (ensure-array #(1.3 "foo")))