diff --git a/index.js b/index.js
index e57f75c6..70792edc 100644
--- a/index.js
+++ b/index.js
@@ -64,11 +64,18 @@ async function fastifyRateLimit (fastify, settings) {
     : defaultMax
 
   // Global time window
-  globalParams.timeWindow = typeof settings.timeWindow === 'string'
-    ? ms.parse(settings.timeWindow)
-    : typeof settings.timeWindow === 'number' && Number.isFinite(settings.timeWindow) && settings.timeWindow >= 0
-      ? Math.trunc(settings.timeWindow)
-      : defaultTimeWindow
+  const twType = typeof settings.timeWindow
+  globalParams.timeWindow = defaultTimeWindow
+  if (twType === 'function') {
+    globalParams.timeWindow = settings.timeWindow
+  } else if (twType === 'string') {
+    globalParams.timeWindow = ms.parse(settings.timeWindow)
+  } else if (
+    twType === 'number' &&
+    Number.isFinite(settings.timeWindow) && settings.timeWindow >= 0
+  ) {
+    globalParams.timeWindow = Math.trunc(settings.timeWindow)
+  }
 
   globalParams.hook = settings.hook || defaultHook
   globalParams.allowList = settings.allowList || settings.whitelist || null
@@ -147,7 +154,7 @@ function mergeParams (...params) {
     result.timeWindow = ms.parse(result.timeWindow)
   } else if (typeof result.timeWindow === 'number' && Number.isFinite(result.timeWindow) && result.timeWindow >= 0) {
     result.timeWindow = Math.trunc(result.timeWindow)
-  } else {
+  } else if (typeof result.timeWindow !== 'function') {
     result.timeWindow = defaultTimeWindow
   }
 
@@ -180,7 +187,6 @@ function addRouteRateHook (pluginComponent, params, routeOptions) {
 
 function rateLimitRequestHandler (pluginComponent, params) {
   const { rateLimitRan, store } = pluginComponent
-  const timeWindowString = ms.format(params.timeWindow, true)
 
   return async (req, res) => {
     if (req[rateLimitRan]) {
@@ -204,6 +210,7 @@ function rateLimitRequestHandler (pluginComponent, params) {
     }
 
     const max = typeof params.max === 'number' ? params.max : await params.max(req, key)
+    const timeWindow = typeof params.timeWindow === 'number' ? params.timeWindow : await params.timeWindow(req, key)
     let current = 0
     let ttl = 0
     let timeLeftInSeconds = 0
@@ -213,7 +220,7 @@ function rateLimitRequestHandler (pluginComponent, params) {
       const res = await new Promise((resolve, reject) => {
         store.incr(key, (err, res) => {
           err ? reject(err) : resolve(res)
-        }, max)
+        }, timeWindow, max)
       })
 
       current = res.current
@@ -248,7 +255,7 @@ function rateLimitRequestHandler (pluginComponent, params) {
       ban: false,
       max,
       ttl,
-      after: timeWindowString
+      after: ms.format(timeWindow, true)
     }
 
     if (params.ban !== -1 && current - max > params.ban) {
diff --git a/store/LocalStore.js b/store/LocalStore.js
index 9fed5006..e3024bbf 100644
--- a/store/LocalStore.js
+++ b/store/LocalStore.js
@@ -4,21 +4,20 @@ const { LruMap: Lru } = require('toad-cache')
 
 function LocalStore (cache = 5000, timeWindow, continueExceeding) {
   this.lru = new Lru(cache)
-  this.timeWindow = timeWindow
   this.continueExceeding = continueExceeding
 }
 
-LocalStore.prototype.incr = function (ip, cb, max) {
+LocalStore.prototype.incr = function (ip, cb, timeWindow, max) {
   const nowInMs = Date.now()
   let current = this.lru.get(ip)
 
   if (!current) {
     // Item doesn't exist
-    current = { current: 1, ttl: this.timeWindow, iterationStartMs: nowInMs }
-  } else if (current.iterationStartMs + this.timeWindow <= nowInMs) {
+    current = { current: 1, ttl: timeWindow, iterationStartMs: nowInMs }
+  } else if (current.iterationStartMs + timeWindow <= nowInMs) {
     // Item has expired
     current.current = 1
-    current.ttl = this.timeWindow
+    current.ttl = timeWindow
     current.iterationStartMs = nowInMs
   } else {
     // Item is alive
@@ -26,10 +25,10 @@ LocalStore.prototype.incr = function (ip, cb, max) {
 
     // Reset TLL if max has been exceeded and `continueExceeding` is enabled
     if (this.continueExceeding && current.current > max) {
-      current.ttl = this.timeWindow
+      current.ttl = timeWindow
       current.iterationStartMs = nowInMs
     } else {
-      current.ttl = this.timeWindow - (nowInMs - current.iterationStartMs)
+      current.ttl = timeWindow - (nowInMs - current.iterationStartMs)
     }
   }
 
diff --git a/store/RedisStore.js b/store/RedisStore.js
index ebef6786..e3c5b2c9 100644
--- a/store/RedisStore.js
+++ b/store/RedisStore.js
@@ -39,8 +39,8 @@ function RedisStore (redis, key = 'fastify-rate-limit-', timeWindow, continueExc
   }
 }
 
-RedisStore.prototype.incr = function (ip, cb, max) {
-  this.redis.rateLimit(this.key + ip, this.timeWindow, max, this.continueExceeding, (err, result) => {
+RedisStore.prototype.incr = function (ip, cb, timeWindow, max) {
+  this.redis.rateLimit(this.key + ip, timeWindow, max, this.continueExceeding, (err, result) => {
     err ? cb(err, null) : cb(null, { current: result[0], ttl: result[1] })
   })
 }
diff --git a/test/global-rate-limit.test.js b/test/global-rate-limit.test.js
index 05a9362d..8bdc0694 100644
--- a/test/global-rate-limit.test.js
+++ b/test/global-rate-limit.test.js
@@ -106,6 +106,54 @@ test('With text timeWindow', async t => {
   })
 })
 
+test('With function timeWindow', async t => {
+  t.plan(15)
+  t.context.clock = FakeTimers.install()
+  const fastify = Fastify()
+  await fastify.register(rateLimit, { max: 2, timeWindow: (_, __) => 1000 })
+
+  fastify.get('/', async (req, reply) => 'hello!')
+
+  let res
+
+  res = await fastify.inject('/')
+
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '1')
+
+  res = await fastify.inject('/')
+
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '0')
+
+  res = await fastify.inject('/')
+
+  t.equal(res.statusCode, 429)
+  t.equal(res.headers['content-type'], 'application/json; charset=utf-8')
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '0')
+  t.equal(res.headers['retry-after'], '1')
+  t.same({
+    statusCode: 429,
+    error: 'Too Many Requests',
+    message: 'Rate limit exceeded, retry in 1 second'
+  }, JSON.parse(res.payload))
+
+  t.context.clock.tick(1100)
+
+  res = await fastify.inject('/')
+
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '1')
+
+  t.teardown(() => {
+    t.context.clock.uninstall()
+  })
+})
+
 test('When passing NaN to the timeWindow property then the timeWindow should be the default value - 60 seconds', async t => {
   t.plan(5)
 
diff --git a/test/route-rate-limit.test.js b/test/route-rate-limit.test.js
index c3029293..2336a5f2 100644
--- a/test/route-rate-limit.test.js
+++ b/test/route-rate-limit.test.js
@@ -92,7 +92,69 @@ test('With text timeWindow', async t => {
   await fastify.register(rateLimit, { global: false })
 
   fastify.get('/', {
-    config: defaultRouteConfig
+    config: {
+      rateLimit: {
+        max: 2,
+        timeWindow: '1s'
+      },
+      someOtherPlugin: {
+        someValue: 1
+      }
+    }
+  }, async (req, reply) => 'hello!')
+
+  let res
+
+  res = await fastify.inject('/')
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '1')
+
+  res = await fastify.inject('/')
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '0')
+
+  res = await fastify.inject('/')
+  t.equal(res.statusCode, 429)
+  t.equal(res.headers['content-type'], 'application/json; charset=utf-8')
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '0')
+  t.equal(res.headers['retry-after'], '1')
+  t.same(JSON.parse(res.payload), {
+    statusCode: 429,
+    error: 'Too Many Requests',
+    message: 'Rate limit exceeded, retry in 1 second'
+  })
+
+  t.context.clock.tick(1100)
+
+  res = await fastify.inject('/')
+  t.equal(res.statusCode, 200)
+  t.equal(res.headers['x-ratelimit-limit'], '2')
+  t.equal(res.headers['x-ratelimit-remaining'], '1')
+
+  t.teardown(() => {
+    t.context.clock.uninstall()
+  })
+})
+
+test('With function timeWindow', async t => {
+  t.plan(15)
+  t.context.clock = FakeTimers.install()
+  const fastify = Fastify()
+  await fastify.register(rateLimit, { global: false })
+
+  fastify.get('/', {
+    config: {
+      rateLimit: {
+        max: 2,
+        timeWindow: (_, __) => 1000
+      },
+      someOtherPlugin: {
+        someValue: 1
+      }
+    }
   }, async (req, reply) => 'hello!')
 
   let res
diff --git a/types/index.d.ts b/types/index.d.ts
index f3ddcf84..b50bc878 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -80,10 +80,14 @@ declare namespace fastifyRateLimit {
 
   export interface RateLimitOptions {
     max?:
-    | number
-    | ((req: FastifyRequest, key: string) => number)
-    | ((req: FastifyRequest, key: string) => Promise<number>);
-    timeWindow?: number | string;
+      | number
+      | ((req: FastifyRequest, key: string) => number)
+      | ((req: FastifyRequest, key: string) => Promise<number>);
+    timeWindow?:
+      | number
+      | string
+      | ((req: FastifyRequest, key: string) => number)
+      | ((req: FastifyRequest, key: string) => Promise<number>);
     hook?: RateLimitHook;
     cache?: number;
     store?: FastifyRateLimitStoreCtor;
diff --git a/types/index.test-d.ts b/types/index.test-d.ts
index e18dc614..5e070a5e 100644
--- a/types/index.test-d.ts
+++ b/types/index.test-d.ts
@@ -114,6 +114,22 @@ const options6: RateLimitPluginOptions = {
   hook: 'preHandler'
 }
 
+const options7: RateLimitPluginOptions = {
+  global: true,
+  max: (req: FastifyRequest<RequestGenericInterface>, key: string) => 42,
+  timeWindow: (req: FastifyRequest<RequestGenericInterface>, key: string) => 5000,
+  store: CustomStore,
+  hook: 'preValidation'
+}
+
+const options8: RateLimitPluginOptions = {
+  global: true,
+  max: (req: FastifyRequest<RequestGenericInterface>, key: string) => 42,
+  timeWindow: (req: FastifyRequest<RequestGenericInterface>, key: string) => Promise.resolve(5000),
+  store: CustomStore,
+  hook: 'preValidation'
+}
+
 appWithImplicitHttp.register(fastifyRateLimit, options1)
 appWithImplicitHttp.register(fastifyRateLimit, options2)
 appWithImplicitHttp.register(fastifyRateLimit, options5)
@@ -144,6 +160,8 @@ appWithHttp2.register(fastifyRateLimit, options1)
 appWithHttp2.register(fastifyRateLimit, options2)
 appWithHttp2.register(fastifyRateLimit, options3)
 appWithHttp2.register(fastifyRateLimit, options5)
+appWithHttp2.register(fastifyRateLimit, options7)
+appWithHttp2.register(fastifyRateLimit, options8)
 
 appWithHttp2.get('/public', {
   config: {