diff --git a/deep-map/index.d.ts b/deep-map/index.d.ts
index 469b522a..6214339c 100644
--- a/deep-map/index.d.ts
+++ b/deep-map/index.d.ts
@@ -1,7 +1,7 @@
 import type { WritableAtom } from '../atom/index.js'
 import type { AllPaths, BaseDeepMap, FromPath } from './path.js'
 
-export { AllPaths, BaseDeepMap, FromPath, getPath, setPath } from './path.js'
+export { AllPaths, BaseDeepMap, FromPath, getPath, setByKey, setPath } from './path.js'
 
 export type DeepMapStore<T extends BaseDeepMap> = {
   /**
diff --git a/deep-map/index.js b/deep-map/index.js
index 44d1081a..024e3958 100644
--- a/deep-map/index.js
+++ b/deep-map/index.js
@@ -1,7 +1,7 @@
 import { atom } from '../atom/index.js'
 import { getPath, setPath } from './path.js'
 
-export { getPath, setPath } from './path.js'
+export { getPath, setByKey, setPath } from './path.js'
 
 export function deepMap(initial = {}) {
   let $deepMap = atom(initial)
diff --git a/deep-map/path.d.ts b/deep-map/path.d.ts
index 99464f5a..57042954 100644
--- a/deep-map/path.d.ts
+++ b/deep-map/path.d.ts
@@ -122,3 +122,25 @@ export function setPath<T extends BaseDeepMap, K extends AllPaths<T>>(
   path: K,
   value: FromPath<T, K>
 ): T
+
+/**
+ * Set a deep value by path. Initialized arrays with `undefined`
+ * if you set arbitrary length.
+ *
+ * ```
+ * import { setByKey } from 'nanostores'
+ *
+ * setByKey({ a: { b: { c: [] } } }, ['a', 'b', 'c', 1], 'hey')
+ * // Returns `{ a: { b: { c: [undefined, 'hey'] } } }`
+ * ```
+ *
+ * @param obj Any object.
+ * @param splittedKeys An array of keys representing the path to the value.
+ * @param value New value.
+ * @retunts The new object.
+ */
+export function setByKey<T extends BaseDeepMap>(
+  obj: T,
+  splittedKeys: PropertyKey[],
+  value: unknown
+): T;
diff --git a/deep-map/path.js b/deep-map/path.js
index c5d5c320..179d7180 100644
--- a/deep-map/path.js
+++ b/deep-map/path.js
@@ -14,7 +14,7 @@ export function setPath(obj, path, value) {
   return setByKey(obj != null ? obj : {}, getAllKeysFromPath(path), value)
 }
 
-function setByKey(obj, splittedKeys, value) {
+export function setByKey(obj, splittedKeys, value) {
   let key = splittedKeys[0]
   ensureKey(obj, key, splittedKeys[1])
   let copy = Array.isArray(obj) ? [...obj] : { ...obj }
diff --git a/index.d.ts b/index.d.ts
index 3c9e16eb..23674387 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -8,6 +8,7 @@ export {
   DeepMapStore,
   FromPath,
   getPath,
+  setByKey,
   setPath
 } from './deep-map/index.js'
 export { keepMount } from './keep-mount/index.js'
diff --git a/index.js b/index.js
index b08a0e7c..ae436664 100644
--- a/index.js
+++ b/index.js
@@ -1,7 +1,7 @@
 export { atom } from './atom/index.js'
 export { clean, cleanStores } from './clean-stores/index.js'
 export { batched, computed } from './computed/index.js'
-export { deepMap, getPath, setPath } from './deep-map/index.js'
+export { deepMap, getPath, setByKey, setPath } from './deep-map/index.js'
 export { keepMount } from './keep-mount/index.js'
 export {
   onMount,