diff --git a/README.md b/README.md
index 99c3cf1..7555fa7 100644
--- a/README.md
+++ b/README.md
@@ -276,6 +276,7 @@ or    | yes | `(or false 0 1)` | `1` | Return the first value that evaluates to
 quote | yes | | | See the section Quoting.
 quasiquote | yes | | | See the section Quoting.
 quasiquote-expand | yes | | | See the section Quoting.
+undef | yes | `(undef myFn)` | `<function>` | Remove a definition from the current environment. Return the removed value.
 
 ## Functions
 
diff --git a/src/Evaller.php b/src/Evaller.php
index 7cc9ad9..cacc8c5 100644
--- a/src/Evaller.php
+++ b/src/Evaller.php
@@ -367,6 +367,16 @@ class Evaller
                     }
 
                     return $this->quasiquote($astData[1]);
+                } elseif ($symbolName == 'undef') {
+                    if ($astLength != 2) {
+                        throw new MadLispException("undef requires exactly 1 argument");
+                    }
+
+                    if (!($astData[1] instanceof Symbol)) {
+                        throw new MadLispException("first argument to undef is not symbol");
+                    }
+
+                    return $env->unset($astData[1]->getName());
                 }
             }