diff --git a/type-definitions/immutable.d.ts b/type-definitions/immutable.d.ts
index db4816ec7..7adfdeb38 100644
--- a/type-definitions/immutable.d.ts
+++ b/type-definitions/immutable.d.ts
@@ -91,6 +91,30 @@
  */
 
 declare namespace Immutable {
+  /**
+   * @ignore
+   */
+  export type DeepCopy<T> = T extends Collection.Keyed<infer KeyedKey, infer V>
+    ? // convert KeyedCollection to DeepCopy plain JS object
+      {
+        [key in KeyedKey extends string | number | symbol
+          ? KeyedKey
+          : string]: DeepCopy<V>;
+      }
+    : // convert IndexedCollection or Immutable.Set to DeepCopy plain JS array
+    T extends Collection<infer _, infer V>
+    ? Array<DeepCopy<V>>
+    : T extends string | number // Iterable scalar types : should be kept as is
+    ? T
+    : T extends Iterable<infer V> // Iterable are converted to plain JS array
+    ? Array<DeepCopy<V>>
+    : T extends object // plain JS object are converted deeply
+    ? {
+        [ObjectKey in keyof T]: DeepCopy<T[ObjectKey]>;
+      }
+    : // other case : should be kept as is
+      T;
+
   /**
    * Lists are ordered indexed dense collections, much like a JavaScript
    * Array.
@@ -2666,7 +2690,7 @@ declare namespace Immutable {
      * Note: This method may not be overridden. Objects with custom
      * serialization to plain JS may override toJSON() instead.
      */
-    toJS(): { [K in keyof TProps]: unknown };
+    toJS(): DeepCopy<TProps>;
 
     /**
      * Shallowly converts this Record to equivalent native JavaScript Object.
@@ -2826,14 +2850,14 @@ declare namespace Immutable {
        *
        * Converts keys to Strings.
        */
-      toJS(): { [key: string]: unknown };
+      toJS(): { [key in string | number | symbol]: DeepCopy<V> };
 
       /**
        * Shallowly converts this Keyed Seq to equivalent native JavaScript Object.
        *
        * Converts keys to Strings.
        */
-      toJSON(): { [key: string]: V };
+      toJSON(): { [key in string | number | symbol]: V };
 
       /**
        * Shallowly converts this collection to an Array.
@@ -2968,7 +2992,7 @@ declare namespace Immutable {
       /**
        * Deeply converts this Indexed Seq to equivalent native JavaScript Array.
        */
-      toJS(): Array<unknown>;
+      toJS(): Array<DeepCopy<T>>;
 
       /**
        * Shallowly converts this Indexed Seq to equivalent native JavaScript Array.
@@ -3143,7 +3167,7 @@ declare namespace Immutable {
       /**
        * Deeply converts this Set Seq to equivalent native JavaScript Array.
        */
-      toJS(): Array<unknown>;
+      toJS(): Array<DeepCopy<T>>;
 
       /**
        * Shallowly converts this Set Seq to equivalent native JavaScript Array.
@@ -3453,14 +3477,14 @@ declare namespace Immutable {
        *
        * Converts keys to Strings.
        */
-      toJS(): { [key: string]: unknown };
+      toJS(): { [key in string | number | symbol]: DeepCopy<V> };
 
       /**
        * Shallowly converts this Keyed collection to equivalent native JavaScript Object.
        *
        * Converts keys to Strings.
        */
-      toJSON(): { [key: string]: V };
+      toJSON(): { [key in string | number | symbol]: V };
 
       /**
        * Shallowly converts this collection to an Array.
@@ -3635,7 +3659,7 @@ declare namespace Immutable {
       /**
        * Deeply converts this Indexed collection to equivalent native JavaScript Array.
        */
-      toJS(): Array<unknown>;
+      toJS(): Array<DeepCopy<T>>;
 
       /**
        * Shallowly converts this Indexed collection to equivalent native JavaScript Array.
@@ -3946,7 +3970,7 @@ declare namespace Immutable {
       /**
        * Deeply converts this Set collection to equivalent native JavaScript Array.
        */
-      toJS(): Array<unknown>;
+      toJS(): Array<DeepCopy<T>>;
 
       /**
        * Shallowly converts this Set collection to equivalent native JavaScript Array.
@@ -4208,7 +4232,9 @@ declare namespace Immutable {
      * `Collection.Indexed`, and `Collection.Set` become `Array`, while
      * `Collection.Keyed` become `Object`, converting keys to Strings.
      */
-    toJS(): Array<unknown> | { [key: string]: unknown };
+    toJS():
+      | Array<DeepCopy<V>>
+      | { [key in string | number | symbol]: DeepCopy<V> };
 
     /**
      * Shallowly converts this Collection to equivalent native JavaScript Array or Object.
@@ -4216,7 +4242,7 @@ declare namespace Immutable {
      * `Collection.Indexed`, and `Collection.Set` become `Array`, while
      * `Collection.Keyed` become `Object`, converting keys to Strings.
      */
-    toJSON(): Array<V> | { [key: string]: V };
+    toJSON(): Array<V> | { [key in string | number | symbol]: V };
 
     /**
      * Shallowly converts this collection to an Array.
diff --git a/type-definitions/ts-tests/deepCopy.ts b/type-definitions/ts-tests/deepCopy.ts
new file mode 100644
index 000000000..3012e0ffc
--- /dev/null
+++ b/type-definitions/ts-tests/deepCopy.ts
@@ -0,0 +1,70 @@
+import { List, Map, Record, Set, Seq, DeepCopy, Collection } from 'immutable';
+
+{
+  // Basic types
+
+  // $ExpectType { a: number; b: number; }
+  type Test = DeepCopy<{ a: number; b: number }>;
+  //   ^?
+
+  // $ExpectType number
+  type TestA = Test['a'];
+  //   ^?
+}
+
+{
+  // Iterables
+
+  // $ExpectType string[]
+  type Test = DeepCopy<string[]>;
+  //   ^?
+
+  // $ExpectType number[]
+  type Keyed = DeepCopy<Collection.Indexed<number>>;
+  //   ^?
+}
+
+{
+  // Immutable first-level types
+
+  // $ExpectType { [x: string]: string; }
+  type StringKey = DeepCopy<Map<string, string>>;
+
+  // $ExpectType { [x: string]: object; }
+  type ObjectKey = DeepCopy<Map<object, object>>;
+
+  // $ExpectType { [x: string]: object; [x: number]: object; }
+  type MixedKey = DeepCopy<Map<object | number, object>>;
+
+  // $ExpectType string[]
+  type ListDeepCopy = DeepCopy<List<string>>;
+
+  // $ExpectType string[]
+  type SetDeepCopy = DeepCopy<Set<string>>;
+}
+
+{
+  // Keyed
+
+  // $ExpectType { [x: string]: number; }
+  type Keyed = DeepCopy<Collection.Keyed<string, number>>;
+
+  // $ExpectType { [x: string]: number; [x: number]: number; }
+  type KeyedMixed = DeepCopy<Collection.Keyed<string | number, number>>;
+
+  // $ExpectType { [x: string]: number; [x: number]: number; }
+  type KeyedSeqMixed = DeepCopy<Seq.Keyed<string | number, number>>;
+
+  // $ExpectType { [x: string]: number; [x: number]: number; }
+  type MapMixed = DeepCopy<Map<string | number, number>>;
+}
+
+{
+  // Nested
+
+  // $ExpectType { map: { [x: string]: string; }; list: string[]; set: string[]; }
+  type NestedObject = DeepCopy<{ map: Map<string, string>; list: List<string>; set: Set<string>; }>;
+
+  // $ExpectType { map: { [x: string]: string; }; }
+  type NestedMap = DeepCopy<Map<'map', Map<string, string>>>;
+}
diff --git a/type-definitions/ts-tests/index.d.ts b/type-definitions/ts-tests/index.d.ts
index 9f2e612b0..11eec1bf4 100644
--- a/type-definitions/ts-tests/index.d.ts
+++ b/type-definitions/ts-tests/index.d.ts
@@ -1,2 +1,2 @@
-// TypeScript Version: 2.2
+// Minimum TypeScript Version: 4.5
 /* tslint:disable:no-useless-files */
diff --git a/type-definitions/ts-tests/list.ts b/type-definitions/ts-tests/list.ts
index eef31b9e9..e1738e9d9 100644
--- a/type-definitions/ts-tests/list.ts
+++ b/type-definitions/ts-tests/list.ts
@@ -445,6 +445,16 @@ import {
   List<number>().asImmutable();
 }
 
+{
+  // #toJS / #toJSON
+
+  // $ExpectType number[][]
+  List<List<number>>().toJS();
+
+  // $ExpectType List<number>[]
+  List<List<number>>().toJSON();
+}
+
 {
   // # for of loops
   const list = List([1, 2, 3, 4]);
diff --git a/type-definitions/ts-tests/map.ts b/type-definitions/ts-tests/map.ts
index ddfe59093..3fffffeda 100644
--- a/type-definitions/ts-tests/map.ts
+++ b/type-definitions/ts-tests/map.ts
@@ -488,3 +488,10 @@ import { Map, List } from 'immutable';
   // $ExpectType Map<number, number>
   Map<number, number>().asImmutable();
 }
+
+{
+  // #toJS
+
+  // $ExpectType { [x: string]: number; [x: number]: number; [x: symbol]: number; }
+  Map<number, number>().toJS();
+}
diff --git a/type-definitions/ts-tests/record.ts b/type-definitions/ts-tests/record.ts
index ae54dc9e0..7a626e438 100644
--- a/type-definitions/ts-tests/record.ts
+++ b/type-definitions/ts-tests/record.ts
@@ -1,4 +1,4 @@
-import { Record } from 'immutable';
+import { List, Map, Record, Set } from 'immutable';
 
 {
   // Factory
@@ -27,6 +27,9 @@ import { Record } from 'immutable';
   // $ExpectError
   pointXY.y = 10;
 
+  // $ExpectType { x: number; y: number; }
+  pointXY.toJS();
+
   class PointClass extends PointXY {
     setX(x: number) {
       return this.set('x', x);
@@ -53,6 +56,12 @@ import { Record } from 'immutable';
 
   // $ExpectType PointClass
   point.setY(10);
+
+  // $ExpectType { x: number; y: number; }
+  point.toJSON();
+
+  // $ExpectType { x: number; y: number; }
+  point.toJS();
 }
 
 {
@@ -65,3 +74,20 @@ import { Record } from 'immutable';
   // $ExpectError
   Record.getDescriptiveName({});
 }
+
+{
+  // Factory
+  const WithMap = Record({
+    map: Map({ a: 'A' }),
+    list: List(['a']),
+    set: Set(['a']),
+  });
+
+  const withMap = WithMap();
+
+  // $ExpectType { map: Map<string, string>; list: List<string>; set: Set<string>; }
+  withMap.toJSON();
+
+  // $ExpectType { map: { [x: string]: string; }; list: string[]; set: string[]; }
+  withMap.toJS();
+}
diff --git a/type-definitions/ts-tests/set.ts b/type-definitions/ts-tests/set.ts
index 3f295a094..596fb0b1b 100644
--- a/type-definitions/ts-tests/set.ts
+++ b/type-definitions/ts-tests/set.ts
@@ -282,3 +282,13 @@ import { Set, Map } from 'immutable';
   // $ExpectType Set<number>
   Set<number>().asImmutable();
 }
+
+{
+  // #toJS / #toJJSON
+
+  // $ExpectType number[][]
+  Set<Set<number>>().toJS();
+
+  // $ExpectType Set<number>[]
+  Set<Set<number>>().toJSON();
+}