diff --git a/index.d.ts b/index.d.ts
index 091d5aa31..4a0d440f6 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -90,4 +90,6 @@ declare namespace dayjs {
   export function extend(plugin: PluginFunc, option?: ConfigType): Dayjs
 
   export function locale(arg1: any, arg2?: any): void
+
+  export function isDayjs(d: any): d is Dayjs
 }
diff --git a/src/index.js b/src/index.js
index 8671018ae..626be49a5 100644
--- a/src/index.js
+++ b/src/index.js
@@ -445,6 +445,8 @@ dayjs.extend = (plugin, option) => {
 
 dayjs.locale = parseLocale
 
+dayjs.isDayjs = isDayjs
+
 dayjs.en = Ls[L]
 
 export default dayjs