diff --git a/packages/runtime-core/src/apiWatch.ts b/packages/runtime-core/src/apiWatch.ts
index 9498ced4..0a6670b2 100644
--- a/packages/runtime-core/src/apiWatch.ts
+++ b/packages/runtime-core/src/apiWatch.ts
@@ -33,6 +33,8 @@ import {
 } from './errorHandling'
 import { queuePostRenderEffect } from './renderer'
 import { warn } from './warning'
+import { DeprecationTypes, warnDeprecation } from './compat/deprecations'
+import { isCompatEnabled } from './compat/compatConfig'
 
 export type WatchEffect = (onInvalidate: InvalidateCbRegistrator) => void
 
@@ -217,6 +219,21 @@ function doWatch(
     __DEV__ && warnInvalidSource(source)
   }
 
+  // 2.x array mutation watch compat
+  if (__COMPAT__ && cb && !deep) {
+    const baseGetter = getter
+    getter = () => {
+      const val = baseGetter()
+      if (isArray(val)) {
+        __DEV__ && warnDeprecation(DeprecationTypes.WATCH_ARRAY)
+        if (isCompatEnabled(DeprecationTypes.WATCH_ARRAY)) {
+          traverse(val)
+        }
+      }
+      return val
+    }
+  }
+
   if (cb && deep) {
     const baseGetter = getter
     getter = () => traverse(baseGetter())
@@ -254,7 +271,14 @@ function doWatch(
     if (cb) {
       // watch(source, cb)
       const newValue = runner()
-      if (deep || forceTrigger || hasChanged(newValue, oldValue)) {
+      if (
+        deep ||
+        forceTrigger ||
+        hasChanged(newValue, oldValue) ||
+        (__COMPAT__ &&
+          isArray(newValue) &&
+          isCompatEnabled(DeprecationTypes.WATCH_ARRAY))
+      ) {
         // cleanup before running cb again
         if (cleanup) {
           cleanup()
diff --git a/packages/runtime-core/src/compat/compatConfig.ts b/packages/runtime-core/src/compat/compatConfig.ts
index f6395883..c430509f 100644
--- a/packages/runtime-core/src/compat/compatConfig.ts
+++ b/packages/runtime-core/src/compat/compatConfig.ts
@@ -16,11 +16,16 @@ export function configureCompat(config: CompatConfig) {
   extend(globalCompatConfig, config)
 }
 
-/**
- * @internal
- */
 export function getCompatConfig(
   key: DeprecationTypes
 ): DeprecationConfigItem | undefined {
   return globalCompatConfig[key]
 }
+
+/**
+ * @internal
+ */
+export function isCompatEnabled(key: DeprecationTypes): boolean {
+  const config = getCompatConfig(key)
+  return !config || config.mode !== 3
+}
diff --git a/packages/runtime-core/src/compat/deprecations.ts b/packages/runtime-core/src/compat/deprecations.ts
index e345a0b2..62c3b906 100644
--- a/packages/runtime-core/src/compat/deprecations.ts
+++ b/packages/runtime-core/src/compat/deprecations.ts
@@ -28,11 +28,10 @@ export const enum DeprecationTypes {
   OPTIONS_BEFORE_DESTROY = 'OPTIONS_BEFORE_DESTROY',
   OPTIONS_DESTROYED = 'OPTIONS_DESTROYED',
 
+  V_ON_KEYCODE_MODIFIER = 'V_ON_KEYCODE_MODIFIER',
   PROPS_DEFAULT_THIS = 'PROPS_DEFAULT_THIS',
-
   CUSTOM_DIR = 'CUSTOM_DIR',
-
-  V_ON_KEYCODE_MODIFIER = 'V_ON_KEYCODE_MODIFIER'
+  WATCH_ARRAY = 'WATCH_ARRAY'
 }
 
 type DeprecationData = {
@@ -181,6 +180,13 @@ const deprecationMessages: Record<DeprecationTypes, DeprecationData> = {
     message: `\`destroyed\` has been renamed to \`unmounted\`.`
   },
 
+  [DeprecationTypes.V_ON_KEYCODE_MODIFIER]: {
+    message:
+      `Using keyCode as v-on modifier is no longer supported. ` +
+      `Use kebab-case key name modifiers instead.`,
+    link: `https://v3.vuejs.org/guide/migration/keycode-modifiers.html`
+  },
+
   [DeprecationTypes.PROPS_DEFAULT_THIS]: {
     message: (key: string) =>
       `props default value function no longer has access to "this". ` +
@@ -195,11 +201,15 @@ const deprecationMessages: Record<DeprecationTypes, DeprecationData> = {
     link: `https://v3.vuejs.org/guide/migration/custom-directives.html`
   },
 
-  [DeprecationTypes.V_ON_KEYCODE_MODIFIER]: {
+  [DeprecationTypes.WATCH_ARRAY]: {
     message:
-      `Using keyCode as v-on modifier is no longer supported. ` +
-      `Use kebab-case key name modifiers instead.`,
-    link: `https://v3.vuejs.org/guide/migration/keycode-modifiers.html`
+      `"watch" option or vm.$watch on an array value will no longer ` +
+      `trigger on array mutation unless the "deep" option is specified. ` +
+      `If current usage is intended, you can suppress this warning with:` +
+      `\n\n  configureCompat({ ${
+        DeprecationTypes.WATCH_ARRAY
+      }: { mode: 3 }})\n`,
+    link: `https://v3.vuejs.org/guide/migration/watch.html`
   }
 }
 
@@ -215,7 +225,10 @@ export function warnDeprecation(key: DeprecationTypes, ...args: any[]) {
 
   // check user config
   const config = getCompatConfig(key)
-  if (config && config.warning === false) {
+  if (
+    config &&
+    (config.warning === false || (config.mode === 3 && config.warning !== true))
+  ) {
     return
   }
 
diff --git a/packages/runtime-core/src/index.ts b/packages/runtime-core/src/index.ts
index 569b51ab..9f8a94cc 100644
--- a/packages/runtime-core/src/index.ts
+++ b/packages/runtime-core/src/index.ts
@@ -288,12 +288,12 @@ export { LegacyConfig } from './compat/globalConfig'
 
 import { warnDeprecation } from './compat/deprecations'
 import { createCompatVue } from './compat/global'
-import { getCompatConfig } from './compat/compatConfig'
+import { isCompatEnabled } from './compat/compatConfig'
 
 const _compatUtils = {
   warnDeprecation,
   createCompatVue,
-  getCompatConfig
+  isCompatEnabled
 }
 
 export const compatUtils = (__COMPAT__