diff --git a/integration/feature/gen-idea/resources/hello-kotlin-idea/build.mill b/integration/feature/gen-idea/resources/hello-kotlin-idea/build.mill
index 7d309e6d3b6..a22b266afb3 100644
--- a/integration/feature/gen-idea/resources/hello-kotlin-idea/build.mill
+++ b/integration/feature/gen-idea/resources/hello-kotlin-idea/build.mill
@@ -5,7 +5,7 @@ import mill.kotlinlib.{KotlinModule, KotlinIdeaModule}
 import mill.scalalib.TestModule
 
 object HelloKotlin extends KotlinModule with KotlinIdeaModule {
-  def kotlinVersion = "2.1.20"
+  def kotlinVersion = "2.1.21"
 
   def kotlinExplicitApi = true
 
@@ -13,7 +13,7 @@ object HelloKotlin extends KotlinModule with KotlinIdeaModule {
 }
 
 object DepKotlin extends KotlinModule with KotlinIdeaModule {
-  def kotlinVersion = "2.1.20"
+  def kotlinVersion = "2.1.21"
   def moduleDeps = Seq(HelloKotlin)
 
   override def kotlinFriendModules: Seq[KotlinModule] = Seq(HelloKotlin)
diff --git a/mill-build/src/millbuild/Deps.scala b/mill-build/src/millbuild/Deps.scala
index 1e5773e43de..4e372f68507 100644
--- a/mill-build/src/millbuild/Deps.scala
+++ b/mill-build/src/millbuild/Deps.scala
@@ -235,7 +235,7 @@ object Deps {
   val requests = mvn"com.lihaoyi::requests:0.9.3"
   val logback = mvn"ch.qos.logback:logback-classic:1.5.33"
   val sonatypeCentralClient = mvn"com.lumidion::sonatype-central-client-requests:0.6.0"
-  val kotlinVersion = "2.1.20"
+  val kotlinVersion = "2.1.21"
   val kspVersion = "2.0.1"
   val kotlinBuildToolsApiVersion_api = "2.3.0"
   val kotlinCompiler = mvn"org.jetbrains.kotlin:kotlin-compiler:$kotlinVersion"