diff --git a/CHANGELOG.md b/CHANGELOG.md
index e3c1527a..6bced219 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 - Fixed type signatures of `Array.fromArrayLikeWithMap` and `Array.fromIteratorWithMap`. https://github.com/rescript-association/rescript-core/pull/50
 - Remove internal async/await helpers that do not need to be exposed in `Core`.
 - Add locale and formatting options to `localeDateString`, `localeString` and `localTimeString` functions https://github.com/rescript-association/rescript-core/pull/30
+- Remove `Undefined` module. `Nullable` or `option` directly (where appropriate) can be used instead. https://github.com/rescript-association/rescript-core/pull/59
 - Change `RegExp.source` to return a `string`. Was previously returning a `bool`, which is wrong. https://github.com/rescript-association/rescript-core/pull/47
 - Remove `Date.valueOf` as it returns the same as `Date.getTime`. https://github.com/rescript-association/rescript-core/pull/61
 
diff --git a/src/Core__Undefined.mjs b/src/Core__Undefined.mjs
deleted file mode 100644
index bdc0e58c..00000000
--- a/src/Core__Undefined.mjs
+++ /dev/null
@@ -1,15 +0,0 @@
-// Generated by ReScript, PLEASE EDIT WITH CARE
-
-import * as Caml_option from "rescript/lib/es6/caml_option.js";
-
-function fromOption(option) {
-  if (option !== undefined) {
-    return Caml_option.valFromOption(option);
-  }
-  
-}
-
-export {
-  fromOption ,
-}
-/* No side effect */
diff --git a/src/Core__Undefined.res b/src/Core__Undefined.res
deleted file mode 100644
index c5a19acf..00000000
--- a/src/Core__Undefined.res
+++ /dev/null
@@ -1,15 +0,0 @@
-type t<'a> = Js.Undefined.t<'a>
-
-external asNullable: t<'a> => Core__Nullable.t<'a> = "%identity"
-
-external undefined: t<'a> = "#undefined"
-
-external make: 'a => t<'a> = "%identity"
-
-external toOption: t<'a> => option<'a> = "#undefined_to_opt"
-
-let fromOption: option<'a> => t<'a> = option =>
-  switch option {
-  | Some(x) => make(x)
-  | None => undefined
-  }
diff --git a/src/Core__Undefined.resi b/src/Core__Undefined.resi
deleted file mode 100644
index 0ffbf5d1..00000000
--- a/src/Core__Undefined.resi
+++ /dev/null
@@ -1,75 +0,0 @@
-/***
-Functions for handling values that could be `undefined`.
-
-Please note that a regular `option` is also represented as `undefined` or the value `'a`. In many cases it makes sense to use `option` directly.
-
-If you also need to cover `null`, check out `Nullable` instead.
-*/
-
-/**
-A type representing a value that can be either `'a` or `undefined`.
-*/
-type t<'a> = Js.Undefined.t<'a>
-
-/**
-Converts a `Undefined.t` into a `Nullable.t`.
-
-## Examples
-```rescript
-let undefinedValue = Undefined.make("Hello")
-let asNullable = undefinedValue->Undefined.asNullable // Nullable.t<string>
-```
-*/
-external asNullable: t<'a> => Core__Nullable.t<'a> = "%identity"
-
-/**
-The value `undefined`.
-
-See [`undefined`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/undefined) on MDN.
-
-## Examples
-```rescript
-Console.log(Undefined.undefined) // Logs `undefined` to the console.
-```
-*/
-external undefined: t<'a> = "#undefined"
-
-/**
-Creates a new `Undefined.t` from the provided value. 
-This means the compiler will enforce `undefined` checks for the new value.
-
-## Examples
-```rescript
-let myStr = "Hello"
-let asUndefinedValue = myStr->Undefined.make // The compiler now thinks this can be `string` or `undefined`.
-```
-*/
-external make: 'a => t<'a> = "%identity"
-
-/**
-Converts a potentially undefined value into an option, so it can be pattern matched on.
-Will convert `undefined` to `None`, and a present value to `Some(value)`.
-
-## Examples
-```rescript
-let undefinedStr = Undefined.make("Hello")
-
-switch undefinedStr->Undefined.toOption {
-| Some(str) => Console.log2("Got string:", str)
-| None => Console.log("Didn't have a value.")
-}
-```
-*/
-external toOption: t<'a> => option<'a> = "#undefined_to_opt"
-
-/**
-Turns an `option` into a `Undefined.t`. `None` will be converted to `undefined`.
-
-## Examples
-```rescript
-let optString: option<string> = None
-let asUndefined = optString->Undefined.fromOption // Undefined.t<string>
-Console.log(asUndefined == Undefined.undefined) // Logs `true` to the console.
-```
-*/
-let fromOption: option<'a> => t<'a>
diff --git a/src/RescriptCore.mjs b/src/RescriptCore.mjs
index 760c8d0d..7706a777 100644
--- a/src/RescriptCore.mjs
+++ b/src/RescriptCore.mjs
@@ -33,8 +33,6 @@ var $$RegExp;
 
 var $$String;
 
-var Undefined;
-
 var $$Symbol;
 
 var Type;
@@ -112,7 +110,6 @@ export {
   $$Promise ,
   $$RegExp ,
   $$String ,
-  Undefined ,
   $$Symbol ,
   Type ,
   $$JSON ,
diff --git a/src/RescriptCore.res b/src/RescriptCore.res
index e8262f17..7cd77db2 100644
--- a/src/RescriptCore.res
+++ b/src/RescriptCore.res
@@ -16,7 +16,6 @@ module Object = Core__Object
 module Promise = Core__Promise
 module RegExp = Core__RegExp
 module String = Core__String
-module Undefined = Core__Undefined
 module Symbol = Core__Symbol
 module Type = Core__Type
 module JSON = Core__JSON
@@ -49,7 +48,7 @@ module Intl = Core__Intl
 @val external globalThis: {..} = "globalThis"
 
 external null: Core__Null.t<'a> = "#null"
-external undefined: Core__Undefined.t<'a> = "#undefined"
+external undefined: Core__Nullable.t<'a> = "#undefined"
 external typeof: 'a => Core__Type.t = "#typeof"
 
 type t<'a> = Js.t<'a>
diff --git a/test/TempTests.mjs b/test/TempTests.mjs
index 6c63e1e9..9807a51a 100644
--- a/test/TempTests.mjs
+++ b/test/TempTests.mjs
@@ -8,7 +8,6 @@ import * as Core__Array from "../src/Core__Array.mjs";
 import * as Core__Float from "../src/Core__Float.mjs";
 import * as Core__BigInt from "../src/Core__BigInt.mjs";
 import * as Core__Option from "../src/Core__Option.mjs";
-import * as Core__Undefined from "../src/Core__Undefined.mjs";
 
 console.info("");
 
@@ -311,8 +310,6 @@ var resolvedOptions = new Intl.DateTimeFormat().resolvedOptions();
 
 var timeZone = resolvedOptions.timeZone;
 
-var x$1 = Core__Undefined.fromOption(1);
-
 var z = 1.2 % 1.4;
 
 var intFromBigInt = Core__BigInt.toInt(BigInt("10000000000"));
@@ -350,11 +347,11 @@ export {
   result$1 as result,
   set ,
   regexp ,
+  x ,
   array$1 as array,
   timeout ,
   resolvedOptions ,
   timeZone ,
-  x$1 as x,
   z ,
   intFromBigInt ,
   Bugfix ,
diff --git a/test/TempTests.res b/test/TempTests.res
index 4c3233b9..82784cea 100644
--- a/test/TempTests.res
+++ b/test/TempTests.res
@@ -194,8 +194,6 @@ if globalThis["hello"] !== undefined {
 let resolvedOptions = Intl.DateTimeFormat.make()->Intl.DateTimeFormat.resolvedOptions
 let timeZone = resolvedOptions["timeZone"]
 
-let x = Some(1)->Undefined.fromOption
-
 let z = Float.mod(1.2, 1.4)
 
 let intFromBigInt = BigInt.fromString("10000000000")->BigInt.toInt