diff --git a/.changeset/observable-map-get-or-insert.md b/.changeset/observable-map-get-or-insert.md
new file mode 100644
index 000000000..d8e72c110
--- /dev/null
+++ b/.changeset/observable-map-get-or-insert.md
@@ -0,0 +1,5 @@
+---
+"mobx": patch
+---
+
+Add `getOrInsert` and `getOrInsertComputed` to `ObservableMap` for compatibility with ESNext `Map` typings.
diff --git a/packages/mobx/__tests__/v5/base/map.js b/packages/mobx/__tests__/v5/base/map.js
index c21b5b11d..31507849b 100644
--- a/packages/mobx/__tests__/v5/base/map.js
+++ b/packages/mobx/__tests__/v5/base/map.js
@@ -700,6 +700,38 @@ test("issue 940, should not be possible to change maps outside strict mode", ()
     }
 })
 
+test("map getOrInsert", function () {
+    const names = map({ user1: "User 1" })
+
+    const values = []
+    const dispose = autorun(() => {
+        values.push(names.getOrInsert("user1", "User 2"))
+    })
+    names.set("user1", "Updated")
+    dispose()
+    expect(values).toEqual(["User 1", "Updated"])
+
+    expect(names.getOrInsert("user2", "User 2")).toBe("User 2")
+    expect(names.get("user2")).toBe("User 2")
+})
+
+test("map getOrInsertComputed", function () {
+    const names = map({ user1: "User 1" })
+    const getName = userId => "User " + userId.slice(4)
+
+    const values = []
+    const dispose = autorun(() => {
+        values.push(names.getOrInsertComputed("user1", getName))
+    })
+    names.set("user1", "Updated")
+    dispose()
+
+    expect(values).toEqual(["User 1", "Updated"])
+
+    expect(names.getOrInsertComputed("user2", getName)).toBe("User 2")
+    expect(names.get("user2")).toBe("User 2")
+})
+
 test("issue 1243, .replace should not trigger change on unchanged values", () => {
     const m = mobx.observable.map({ a: 1, b: 2, c: 3 })
 
diff --git a/packages/mobx/src/types/observablemap.ts b/packages/mobx/src/types/observablemap.ts
index 014b65507..15061103a 100644
--- a/packages/mobx/src/types/observablemap.ts
+++ b/packages/mobx/src/types/observablemap.ts
@@ -289,6 +289,20 @@ export class ObservableMap<K = any, V = any>
         return this.dehanceValue_(undefined)
     }
 
+    getOrInsert(key: K, value: V): V {
+        if (!this.has(key)) {
+            this.set(key, value)
+        }
+        return this.get(key)!
+    }
+
+    getOrInsertComputed(key: K, callback: (key: K) => V): V {
+        if (!this.has(key)) {
+            this.set(key, callback(key))
+        }
+        return this.get(key)!
+    }
+
     private dehanceValue_<X extends V | undefined>(value: X): X {
         if (this.dehancer !== undefined) {
             return this.dehancer(value)