diff --git a/dtslint/ts3.5/Encoder.ts b/dtslint/ts3.5/Encoder.ts
index 8bb901d4..d5911e13 100644
--- a/dtslint/ts3.5/Encoder.ts
+++ b/dtslint/ts3.5/Encoder.ts
@@ -26,6 +26,11 @@ export type OfTestOutput = E.OutputOf<typeof OfTest> // $ExpectType { a: string;
 //
 E.nullable(NumberToString) // $ExpectType Encoder<string | null, number | null>
 
+//
+// optional
+//
+E.optional(NumberToString) // $ExpectType Encoder<string | undefined, number | undefined>
+
 //
 // struct
 //
diff --git a/dtslint/ts3.5/Schema.ts b/dtslint/ts3.5/Schema.ts
index ee8729eb..da570ac7 100644
--- a/dtslint/ts3.5/Schema.ts
+++ b/dtslint/ts3.5/Schema.ts
@@ -53,6 +53,12 @@ make((S) => S.boolean) // $ExpectType Schema<boolean>
 
 make((S) => S.nullable(S.string)) // $ExpectType Schema<string | null>
 
+//
+// optional
+//
+
+make((S) => S.optional(S.string)) // $ExpectType Schema<string | undefined>
+
 //
 // struct
 //
diff --git a/src/Codec.ts b/src/Codec.ts
index 9465e02e..7d589b0d 100644
--- a/src/Codec.ts
+++ b/src/Codec.ts
@@ -142,6 +142,14 @@ export function nullable<I, O, A>(or: Codec<I, O, A>): Codec<null | I, null | O,
   return make(D.nullable(or), E.nullable(or))
 }
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export function optional<I, O, A>(or: Codec<I, O, A>): Codec<undefined | I, undefined | O, undefined | A> {
+  return make(D.optional(or), E.optional(or))
+}
+
 /**
  * @category combinators
  * @since 2.2.15
diff --git a/src/Decoder.ts b/src/Decoder.ts
index 702cc9bb..f626fb63 100644
--- a/src/Decoder.ts
+++ b/src/Decoder.ts
@@ -228,6 +228,14 @@ export const nullable: <I, A>(or: Decoder<I, A>) => Decoder<null | I, null | A>
 
 /**
  * @category combinators
+ * @since 2.3.0
+ */
+export const optional: <I, A>(or: Decoder<I, A>) => Decoder<undefined | I, undefined | A> =
+  /*#__PURE__*/
+  K.optional(M)((u, e) => FS.concat(FS.of(DE.member(0, error(u, 'undefined'))), FS.of(DE.member(1, e))))
+
+  /**
+ * @category combinators
  * @since 2.2.15
  */
 export const fromStruct = <P extends Record<string, Decoder<any, any>>>(
@@ -488,6 +496,7 @@ export const Schemable: S.Schemable2C<URI, unknown> = {
   number,
   boolean,
   nullable,
+  optional,
   type,
   struct,
   partial,
diff --git a/src/Encoder.ts b/src/Encoder.ts
index 8cf7a085..2dc41141 100644
--- a/src/Encoder.ts
+++ b/src/Encoder.ts
@@ -39,6 +39,16 @@ export function nullable<O, A>(or: Encoder<O, A>): Encoder<null | O, null | A> {
   }
 }
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export function optional<O, A>(or: Encoder<O, A>): Encoder<undefined | O, undefined | A> {
+  return {
+    encode: (a) => (a === undefined ? undefined : or.encode(a))
+  }
+}
+
 /**
  * @category combinators
  * @since 2.2.15
diff --git a/src/Eq.ts b/src/Eq.ts
index 67015156..c8062775 100644
--- a/src/Eq.ts
+++ b/src/Eq.ts
@@ -89,6 +89,16 @@ export function nullable<A>(or: Eq<A>): Eq<null | A> {
   }
 }
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export function optional<A>(or: Eq<A>): Eq<undefined | A> {
+  return {
+    equals: (x, y) => (x === undefined || y === undefined ? x === y : or.equals(x, y))
+  }
+}
+
 /**
  * @category combinators
  * @since 2.2.15
@@ -204,6 +214,7 @@ export const Schemable: Schemable1<E.URI> = {
   number,
   boolean,
   nullable,
+  optional,
   type,
   struct,
   partial,
diff --git a/src/Guard.ts b/src/Guard.ts
index d189660f..741adc63 100644
--- a/src/Guard.ts
+++ b/src/Guard.ts
@@ -120,6 +120,14 @@ export const nullable = <I, A extends I>(or: Guard<I, A>): Guard<null | I, null
   is: (i): i is null | A => i === null || or.is(i)
 })
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export const optional = <I, A extends I>(or: Guard<I, A>): Guard<undefined | I, undefined | A> => ({
+  is: (i): i is undefined | A => i === undefined || or.is(i)
+})
+
 /**
  * @category combinators
  * @since 2.2.15
@@ -325,6 +333,7 @@ export const Schemable: S.Schemable1<URI> = {
   number,
   boolean,
   nullable,
+  optional,
   type,
   struct,
   partial,
diff --git a/src/Kleisli.ts b/src/Kleisli.ts
index b6cba277..c40a993c 100644
--- a/src/Kleisli.ts
+++ b/src/Kleisli.ts
@@ -123,6 +123,28 @@ export function nullable<M extends URIS2, E>(
   })
 }
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export function optional<M extends URIS2, E>(
+  M: Applicative2C<M, E> & Bifunctor2<M>
+): <I>(
+  onError: (i: I, e: E) => E
+) => <A>(or: Kleisli<M, I, E, A>) => Kleisli<M, undefined | I, E, undefined | A> {
+  return <I>(onError: (i: I, e: E) => E) =>
+    <A>(or: Kleisli<M, I, E, A>): Kleisli<M, undefined | I, E, undefined | A> => ({
+      decode: (i) =>
+        i === undefined
+          ? M.of<undefined | A>(undefined)
+          : M.bimap(
+              or.decode(i),
+              (e) => onError(i, e),
+              (a): A | undefined => a
+            ),
+    });
+}
+
 /**
  * @category combinators
  * @since 2.2.15
diff --git a/src/Schemable.ts b/src/Schemable.ts
index 1eda7c3c..5a654537 100644
--- a/src/Schemable.ts
+++ b/src/Schemable.ts
@@ -28,6 +28,7 @@ export interface Schemable<S> {
   readonly number: HKT<S, number>
   readonly boolean: HKT<S, boolean>
   readonly nullable: <A>(or: HKT<S, A>) => HKT<S, null | A>
+  readonly optional: <A>(or: HKT<S, A>) => HKT<S, undefined | A>
   /** @deprecated */
   readonly type: <A>(properties: { [K in keyof A]: HKT<S, A[K]> }) => HKT<S, { [K in keyof A]: A[K] }>
   readonly struct: <A>(properties: { [K in keyof A]: HKT<S, A[K]> }) => HKT<S, { [K in keyof A]: A[K] }>
@@ -55,6 +56,7 @@ export interface Schemable1<S extends URIS> {
   readonly number: Kind<S, number>
   readonly boolean: Kind<S, boolean>
   readonly nullable: <A>(or: Kind<S, A>) => Kind<S, null | A>
+  readonly optional: <A>(or: Kind<S, A>) => Kind<S, undefined | A>
   /** @deprecated */
   readonly type: <A>(properties: { [K in keyof A]: Kind<S, A[K]> }) => Kind<S, { [K in keyof A]: A[K] }>
   readonly struct: <A>(properties: { [K in keyof A]: Kind<S, A[K]> }) => Kind<S, { [K in keyof A]: A[K] }>
@@ -82,6 +84,7 @@ export interface Schemable2C<S extends URIS2, E> {
   readonly number: Kind2<S, E, number>
   readonly boolean: Kind2<S, E, boolean>
   readonly nullable: <A>(or: Kind2<S, E, A>) => Kind2<S, E, null | A>
+  readonly optional: <A>(or: Kind2<S, E, A>) => Kind2<S, E, undefined | A>
   /** @deprecated */
   readonly type: <A>(properties: { [K in keyof A]: Kind2<S, E, A[K]> }) => Kind2<S, E, { [K in keyof A]: A[K] }>
   readonly struct: <A>(properties: { [K in keyof A]: Kind2<S, E, A[K]> }) => Kind2<S, E, { [K in keyof A]: A[K] }>
diff --git a/src/TaskDecoder.ts b/src/TaskDecoder.ts
index 0af1501a..4802f4da 100644
--- a/src/TaskDecoder.ts
+++ b/src/TaskDecoder.ts
@@ -231,6 +231,14 @@ export const nullable: <I, A>(or: TaskDecoder<I, A>) => TaskDecoder<null | I, nu
 
 /**
  * @category combinators
+ * @since 2.2.7
+ */
+export const optional: <I, A>(or: TaskDecoder<I, A>) => TaskDecoder<undefined | I, undefined | A> =
+  /*#__PURE__*/
+  K.optional(M)((u, e) => FS.concat(FS.of(DE.member(0, error(u, 'undefined'))), FS.of(DE.member(1, e))))
+
+  /**
+ * @category combinators
  * @since 2.2.15
  */
 export const fromStruct = <P extends Record<string, TaskDecoder<any, any>>>(
@@ -494,6 +502,7 @@ export const Schemable: S.Schemable2C<URI, unknown> = {
   number,
   boolean,
   nullable,
+  optional,
   type,
   struct,
   partial,
diff --git a/src/Type.ts b/src/Type.ts
index 26ab3f20..eded9dd4 100644
--- a/src/Type.ts
+++ b/src/Type.ts
@@ -97,6 +97,12 @@ export const refine = <A, B extends A>(refinement: Refinement<A, B>, id: string)
  */
 export const nullable = <A>(or: Type<A>): Type<null | A> => t.union([t.null, or])
 
+/**
+ * @category combinators
+ * @since 2.3.0
+ */
+export const optional = <A>(or: Type<A>): Type<undefined | A> => t.union([t.undefined, or])
+
 /**
  * @category combinators
  * @since 2.2.15
@@ -206,6 +212,7 @@ export const Schemable: S.Schemable1<URI> = {
   number,
   boolean,
   nullable,
+  optional,
   type,
   struct,
   partial,
diff --git a/test/Arbitrary.ts b/test/Arbitrary.ts
index 0eccbfa9..2496ce14 100644
--- a/test/Arbitrary.ts
+++ b/test/Arbitrary.ts
@@ -52,6 +52,10 @@ export function nullable<A>(or: Arbitrary<A>): Arbitrary<null | A> {
   return fc.oneof(fc.constant(null), or)
 }
 
+export function optional<A>(or: Arbitrary<A>): Arbitrary<undefined | A> {
+  return fc.oneof(fc.constant(undefined), or);
+}
+
 export function struct<A>(properties: { [K in keyof A]: Arbitrary<A[K]> }): Arbitrary<A> {
   return fc.record(properties)
 }
@@ -125,6 +129,7 @@ export const Schemable: S.Schemable1<URI> & S.WithUnknownContainers1<URI> & S.Wi
   number,
   boolean,
   nullable,
+  optional,
   type: struct,
   struct,
   partial,
diff --git a/test/Codec.ts b/test/Codec.ts
index 8b228ee3..a781bff1 100644
--- a/test/Codec.ts
+++ b/test/Codec.ts
@@ -205,6 +205,46 @@ describe('Codec', () => {
     })
   })
 
+  describe("optional", () => {
+    describe("decode", () => {
+      it("should decode a valid input", () => {
+        const codec = _.optional(codecNumber);
+        assert.deepStrictEqual(codec.decode(undefined), D.success(undefined));
+        assert.deepStrictEqual(codec.decode("1"), D.success(1));
+      });
+
+      it("should reject an invalid input", () => {
+        const codec = _.optional(codecNumber);
+        assert.deepStrictEqual(
+          codec.decode(undefined),
+          E.left(
+            FS.concat(
+              FS.of(DE.member(0, FS.of(DE.leaf(undefined, "undefined")))),
+              FS.of(DE.member(1, FS.of(DE.leaf(undefined, "string"))))
+            )
+          )
+        );
+        assert.deepStrictEqual(
+          codec.decode("a"),
+          E.left(
+            FS.concat(
+              FS.of(DE.member(0, FS.of(DE.leaf("a", "undefined")))),
+              FS.of(DE.member(1, FS.of(DE.leaf("a", "parsable to a number"))))
+            )
+          )
+        );
+      });
+    });
+
+    describe("encode", () => {
+      it("should encode a value", () => {
+        const codec = _.optional(codecNumber);
+        assert.strictEqual(codec.encode(undefined), undefined);
+        assert.strictEqual(codec.encode(1), "1");
+      });
+    });
+  });
+
   describe('struct', () => {
     describe('decode', () => {
       it('should decode a valid input', () => {
diff --git a/test/Decoder.ts b/test/Decoder.ts
index a7e82449..d56833a7 100644
--- a/test/Decoder.ts
+++ b/test/Decoder.ts
@@ -153,6 +153,36 @@ describe('Decoder', () => {
     })
   })
 
+  describe("optional", () => {
+    it("should decode a valid input", () => {
+      const decoder = _.optional(H.decoderNumberFromUnknownString);
+      assert.deepStrictEqual(decoder.decode(undefined), _.success(undefined));
+      assert.deepStrictEqual(decoder.decode("1"), _.success(1));
+    });
+
+    it("should reject an invalid input", () => {
+      const decoder = _.optional(H.decoderNumberFromUnknownString);
+      assert.deepStrictEqual(
+        decoder.decode(undefined),
+        E.left(
+          FS.concat(
+            FS.of(DE.member(0, FS.of(DE.leaf(undefined, "undefined")))),
+            FS.of(DE.member(1, FS.of(DE.leaf(undefined, "string"))))
+          )
+        )
+      );
+      assert.deepStrictEqual(
+        decoder.decode("a"),
+        E.left(
+          FS.concat(
+            FS.of(DE.member(0, FS.of(DE.leaf("a", "undefined")))),
+            FS.of(DE.member(1, FS.of(DE.leaf("a", "parsable to a number"))))
+          )
+        )
+      );
+    });
+  });
+
   describe('struct', () => {
     it('should decode a valid input', async () => {
       const decoder = _.struct({
diff --git a/test/Encoder.ts b/test/Encoder.ts
index d4155a63..0083ce73 100644
--- a/test/Encoder.ts
+++ b/test/Encoder.ts
@@ -20,6 +20,12 @@ describe('Encoder', () => {
     assert.deepStrictEqual(encoder.encode(null), null)
   })
 
+  it("optional", () => {
+    const encoder = E.optional(H.encoderNumberToString);
+    assert.deepStrictEqual(encoder.encode(1), "1");
+    assert.deepStrictEqual(encoder.encode(undefined), undefined);
+  });
+
   it('struct', () => {
     const encoder = E.struct({ a: H.encoderNumberToString, b: H.encoderBooleanToNumber })
     assert.deepStrictEqual(encoder.encode({ a: 1, b: true }), { a: '1', b: 1 })
diff --git a/test/Guard.ts b/test/Guard.ts
index 23cd4318..5b4545c0 100644
--- a/test/Guard.ts
+++ b/test/Guard.ts
@@ -79,6 +79,19 @@ describe('Guard', () => {
     })
   })
 
+  describe("optional", () => {
+    it("should accept valid inputs", () => {
+      const guard = G.optional(G.string);
+      assert.strictEqual(guard.is(undefined), true)
+      assert.strictEqual(guard.is("a"), true)
+    })
+
+    it("should reject invalid inputs", () => {
+      const guard = G.optional(G.string);
+      assert.strictEqual(guard.is(1), false)
+    })
+  })
+
   describe('struct', () => {
     it('should accept valid inputs', () => {
       const guard = G.struct({ a: G.string, b: G.number })
diff --git a/test/Schema.ts b/test/Schema.ts
index 08890a6a..87a61178 100644
--- a/test/Schema.ts
+++ b/test/Schema.ts
@@ -38,6 +38,10 @@ describe('Schema', () => {
     check(make((S) => S.nullable(S.string)))
   })
 
+  it("optional", () => {
+    check(make((S) => S.optional(S.string)))
+  })
+
   it('struct', () => {
     check(
       make((S) =>
diff --git a/test/TaskDecoder.ts b/test/TaskDecoder.ts
index a944df23..7e8f843c 100644
--- a/test/TaskDecoder.ts
+++ b/test/TaskDecoder.ts
@@ -196,6 +196,36 @@ describe('UnknownTaskDecoder', () => {
     })
   })
 
+  describe("optional", () => {
+    it("should decode a valid input", async () => {
+      const decoder = _.optional(NumberFromString);
+      assert.deepStrictEqual(await decoder.decode(undefined)(), D.success(undefined));
+      assert.deepStrictEqual(await decoder.decode("1")(), D.success(1));
+    });
+
+    it("should reject an invalid input", async () => {
+      const decoder = _.optional(NumberFromString);
+      assert.deepStrictEqual(
+        await decoder.decode(undefined)(),
+        E.left(
+          FS.concat(
+            FS.of(DE.member(0, FS.of(DE.leaf(undefined, "undefined")))),
+            FS.of(DE.member(1, FS.of(DE.leaf(undefined, "string"))))
+          )
+        )
+      );
+      assert.deepStrictEqual(
+        await decoder.decode("a")(),
+        E.left(
+          FS.concat(
+            FS.of(DE.member(0, FS.of(DE.leaf("a", "undefined")))),
+            FS.of(DE.member(1, FS.of(DE.leaf("a", "parsable to a number"))))
+          )
+        )
+      );
+    });
+  });
+
   describe('struct', () => {
     it('should decode a valid input', async () => {
       const decoder = _.struct({
diff --git a/test/Type.ts b/test/Type.ts
index 567fe097..dc2325b0 100644
--- a/test/Type.ts
+++ b/test/Type.ts
@@ -105,6 +105,13 @@ describe('Type', () => {
     )
   })
 
+  it("optional", () => {
+    check(
+      make((S) => S.optional(S.string)),
+      t.union([t.undefined, t.string])
+    );
+  });
+
   it('struct', () => {
     check(
       make((S) =>