diff --git a/build-support/build.gradle.kts b/build-support/build.gradle.kts
index a2cdb9ee63..52e0708ceb 100644
--- a/build-support/build.gradle.kts
+++ b/build-support/build.gradle.kts
@@ -81,7 +81,7 @@ allprojects {
   tasks.withType<KotlinCompile>().configureEach {
     compilerOptions {
       jvmTarget.set(JvmTarget.JVM_11)
-      freeCompilerArgs.add("-Xjvm-default=all")
+      freeCompilerArgs.add("-jvm-default=no-compatibility")
     }
   }
 }
diff --git a/build-support/src/main/kotlin/com/squareup/wire/buildsupport/WireBuildPlugin.kt b/build-support/src/main/kotlin/com/squareup/wire/buildsupport/WireBuildPlugin.kt
index 01eb44e881..1feb9747b9 100644
--- a/build-support/src/main/kotlin/com/squareup/wire/buildsupport/WireBuildPlugin.kt
+++ b/build-support/src/main/kotlin/com/squareup/wire/buildsupport/WireBuildPlugin.kt
@@ -50,7 +50,6 @@ import org.gradle.kotlin.dsl.register
 import org.gradle.kotlin.dsl.withType
 import org.jetbrains.dokka.gradle.DokkaExtension
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
-import org.jetbrains.kotlin.gradle.dsl.KotlinJsProjectExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinProjectExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
@@ -231,7 +230,7 @@ class WireBuildPlugin : Plugin<Project> {
     tasks.withType(KotlinJvmCompile::class.java).configureEach {
       compilerOptions {
         jvmTarget.set(JvmTarget.fromTarget(javaVersion.toString()))
-        freeCompilerArgs.add("-Xjvm-default=all")
+        freeCompilerArgs.add("-jvm-default=no-compatibility")
       }
     }
     // Kotlin requires the Java compatibility matches.
@@ -252,48 +251,46 @@ class WireBuildPlugin : Plugin<Project> {
       kotlin.configureWebKotlinLibraries()
     }
     plugins.withId("org.jetbrains.kotlin.js") {
-      val kotlin = extensions.getByName("kotlin") as KotlinJsProjectExtension
-      kotlin.configureWebKotlinLibraries()
+      val kotlin = extensions.getByName("kotlin") as KotlinProjectExtension
+      kotlin.configureJsKotlinLibraries()
     }
   }
 
   // For KotlinWasm/Js, versions of toolchain and stdlib need to be the same:
   // https://youtrack.jetbrains.com/issue/KT-71032
-  private fun KotlinProjectExtension.configureWebKotlinLibraries() {
+  private fun KotlinProjectExtension.configureJsKotlinLibraries() {
     val kotlinVersion = project.getVersionByName("kotlin")
 
-    when (this) {
-      is KotlinJsProjectExtension -> {
-        val suffix = "js"
-        sourceSets.apply {
-          getByName("main").dependencies {
-            implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
-          }
-          getByName("test").dependencies {
-            implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
-            implementation("org.jetbrains.kotlin:kotlin-test-$suffix:$kotlinVersion")
-          }
-        }
+    val suffix = "js"
+    sourceSets.apply {
+      getByName("main").dependencies {
+        implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
       }
+      getByName("test").dependencies {
+        implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
+        implementation("org.jetbrains.kotlin:kotlin-test-$suffix:$kotlinVersion")
+      }
+    }
+  }
 
-      is KotlinMultiplatformExtension -> {
-        targets.matching { it.platformType in listOf(KotlinPlatformType.js, KotlinPlatformType.wasm) }.configureEach {
-          val suffix = when (platformType) {
-            KotlinPlatformType.js -> "js"
-            KotlinPlatformType.wasm -> if (targetName.contains("wasi", true)) "wasm-wasi" else "wasm-js"
-            else -> return@configureEach
-          }
+  private fun KotlinMultiplatformExtension.configureWebKotlinLibraries() {
+    val kotlinVersion = project.getVersionByName("kotlin")
 
-          this@configureWebKotlinLibraries.sourceSets.apply {
-            getByName("${targetName}Main").dependencies {
-              implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
-            }
+    targets.matching { it.platformType in listOf(KotlinPlatformType.js, KotlinPlatformType.wasm) }.configureEach {
+      val suffix = when (platformType) {
+        KotlinPlatformType.js -> "js"
+        KotlinPlatformType.wasm -> if (targetName.contains("wasi", true)) "wasm-wasi" else "wasm-js"
+        else -> return@configureEach
+      }
 
-            getByName("${targetName}Test").dependencies {
-              implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
-              implementation("org.jetbrains.kotlin:kotlin-test-$suffix:$kotlinVersion")
-            }
-          }
+      this@configureWebKotlinLibraries.sourceSets.apply {
+        getByName("${targetName}Main").dependencies {
+          implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
+        }
+
+        getByName("${targetName}Test").dependencies {
+          implementation("org.jetbrains.kotlin:kotlin-stdlib-$suffix:$kotlinVersion")
+          implementation("org.jetbrains.kotlin:kotlin-test-$suffix:$kotlinVersion")
         }
       }
     }
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index d34b443904..8018f4ea60 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -13,7 +13,7 @@ jimfs = "1.3.1"
 jmh = "1.37"
 jsr305 = "3.0.2"
 junit = "4.13.2"
-kotlin = "2.3.21"
+kotlin = "2.4.0"
 # Set to lower version than KGP version to maximize compatibility
 kotlinCoreLibrariesVersion = "2.0.21"
 kotlinpoet = "2.3.0"
diff --git a/samples/js/build.gradle.kts b/samples/js/build.gradle.kts
index 053722d0e0..a413b8ce4f 100644
--- a/samples/js/build.gradle.kts
+++ b/samples/js/build.gradle.kts
@@ -1,5 +1,5 @@
 plugins {
-  kotlin("js")
+  kotlin("multiplatform")
   id("com.squareup.wire")
 }
 
diff --git a/samples/wire-grpc-sample/server/build.gradle.kts b/samples/wire-grpc-sample/server/build.gradle.kts
index 1dcfc165ed..7b672374d7 100644
--- a/samples/wire-grpc-sample/server/build.gradle.kts
+++ b/samples/wire-grpc-sample/server/build.gradle.kts
@@ -19,7 +19,7 @@ tasks.withType<JavaCompile>().configureEach {
 tasks.withType<KotlinCompile>().configureEach {
   compilerOptions {
     jvmTarget.set(JvmTarget.JVM_17)
-    freeCompilerArgs.add("-Xjvm-default=all")
+    freeCompilerArgs.add("-jvm-default=no-compatibility")
   }
 }
 
diff --git a/wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt b/wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt
index fbff3d7eba..986851ebcf 100644
--- a/wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt
+++ b/wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt
@@ -28,8 +28,8 @@ import org.gradle.api.Project
 import org.gradle.api.artifacts.UnknownConfigurationException
 import org.gradle.api.tasks.SourceSet
 import org.gradle.api.tasks.SourceSetContainer
-import org.jetbrains.kotlin.gradle.dsl.KotlinJsProjectExtension
 import org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
+import org.jetbrains.kotlin.gradle.dsl.KotlinProjectExtension
 import org.jetbrains.kotlin.gradle.plugin.sources.DefaultKotlinSourceSet
 
 class WirePlugin : Plugin<Project> {
@@ -270,7 +270,7 @@ class WirePlugin : Plugin<Project> {
 
       isJsOnly -> {
         val sourceSets =
-          project.extensions.getByType(KotlinJsProjectExtension::class.java).sourceSets
+          project.extensions.getByType(KotlinProjectExtension::class.java).sourceSets
         val sourceSet = (sourceSets.getByName("main") as DefaultKotlinSourceSet)
         project.dependencies.add(sourceSet.apiConfigurationName, runtimeDependency)
       }
diff --git a/wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt b/wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt
index 5565e19e48..8d09a92a91 100644
--- a/wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt
+++ b/wire-gradle-plugin/src/test/kotlin/com/squareup/wire/gradle/WirePluginTest.kt
@@ -1266,7 +1266,7 @@ class WirePluginTest {
     }
     assertThat(buildCacheDir.exists()).isFalse()
 
-    val generatedProto = "build/generated/source/wire/com/squareup/geology/Period.kt"
+    val generatedProto = "build/generated/source/wire/com/squareup/geology/Period.java"
 
     val fixtureRoot = File("src/test/projects/cache-relocation-1")
     val result = fixtureGradleRunner(fixtureRoot)
@@ -1319,7 +1319,7 @@ class WirePluginTest {
     }
     assertThat(buildCacheDir.exists()).isFalse()
 
-    val generatedStatus = "build/generated/source/wire/com/google/rpc/Status.kt"
+    val generatedStatus = "build/generated/source/wire/com/google/rpc/Status.java"
 
     val fixtureRoot = File("src/test/projects/cache-source-relocation-1")
     val result = fixtureGradleRunner(fixtureRoot)
diff --git a/wire-gradle-plugin/src/test/projects/cache-relocation-1/build.gradle b/wire-gradle-plugin/src/test/projects/cache-relocation-1/build.gradle
index c33f6c7b57..d6c72cb929 100644
--- a/wire-gradle-plugin/src/test/projects/cache-relocation-1/build.gradle
+++ b/wire-gradle-plugin/src/test/projects/cache-relocation-1/build.gradle
@@ -1,7 +1,6 @@
 buildscript {
   dependencies {
     classpath "com.squareup.wire:wire-gradle-plugin:$wireVersion"
-    classpath libs.pluginz.kotlin
   }
 
   repositories {
@@ -14,7 +13,6 @@ buildscript {
 }
 
 apply plugin: 'application'
-apply plugin: 'org.jetbrains.kotlin.jvm'
 apply plugin: 'com.squareup.wire'
 
 // The code in this test project should be identical to
@@ -35,6 +33,6 @@ dependencies {
 }
 
 wire {
-  kotlin {
+  java {
   }
 }
diff --git a/wire-gradle-plugin/src/test/projects/cache-relocation-2/build.gradle b/wire-gradle-plugin/src/test/projects/cache-relocation-2/build.gradle
index 09df7f5971..5a63d86e86 100644
--- a/wire-gradle-plugin/src/test/projects/cache-relocation-2/build.gradle
+++ b/wire-gradle-plugin/src/test/projects/cache-relocation-2/build.gradle
@@ -1,7 +1,6 @@
 buildscript {
   dependencies {
     classpath "com.squareup.wire:wire-gradle-plugin:$wireVersion"
-    classpath libs.pluginz.kotlin
   }
 
   repositories {
@@ -14,7 +13,6 @@ buildscript {
 }
 
 apply plugin: 'application'
-apply plugin: 'org.jetbrains.kotlin.jvm'
 apply plugin: 'com.squareup.wire'
 
 // The code in this test project should be identical to
@@ -35,6 +33,6 @@ dependencies {
 }
 
 wire {
-  kotlin {
+  java {
   }
 }
diff --git a/wire-gradle-plugin/src/test/projects/cache-source-relocation-1/build.gradle b/wire-gradle-plugin/src/test/projects/cache-source-relocation-1/build.gradle
index 16a5bec382..d25aecd8ab 100644
--- a/wire-gradle-plugin/src/test/projects/cache-source-relocation-1/build.gradle
+++ b/wire-gradle-plugin/src/test/projects/cache-source-relocation-1/build.gradle
@@ -1,7 +1,6 @@
 buildscript {
   dependencies {
     classpath "com.squareup.wire:wire-gradle-plugin:$wireVersion"
-    classpath libs.pluginz.kotlin
   }
 
   repositories {
@@ -14,7 +13,6 @@ buildscript {
 }
 
 apply plugin: 'application'
-apply plugin: 'org.jetbrains.kotlin.jvm'
 apply plugin: 'com.squareup.wire'
 
 // The code in this test project should be identical to
@@ -36,6 +34,6 @@ wire {
     srcJar("com.google.api.grpc:proto-google-common-protos:2.57.0")
     include("google/rpc/status.proto")
   }
-  kotlin {
+  java {
   }
 }
diff --git a/wire-gradle-plugin/src/test/projects/cache-source-relocation-2/build.gradle b/wire-gradle-plugin/src/test/projects/cache-source-relocation-2/build.gradle
index 36b0c36863..21bc96026d 100644
--- a/wire-gradle-plugin/src/test/projects/cache-source-relocation-2/build.gradle
+++ b/wire-gradle-plugin/src/test/projects/cache-source-relocation-2/build.gradle
@@ -1,7 +1,6 @@
 buildscript {
   dependencies {
     classpath "com.squareup.wire:wire-gradle-plugin:$wireVersion"
-    classpath libs.pluginz.kotlin
   }
 
   repositories {
@@ -14,7 +13,6 @@ buildscript {
 }
 
 apply plugin: 'application'
-apply plugin: 'org.jetbrains.kotlin.jvm'
 apply plugin: 'com.squareup.wire'
 
 // The code in this test project should be identical to
@@ -36,6 +34,6 @@ wire {
     srcJar("com.google.api.grpc:proto-google-common-protos:2.57.0")
     include("google/rpc/status.proto")
   }
-  kotlin {
+  java {
   }
 }
diff --git a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Service.kt b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Service.kt
index 87d339381f..517f9e44c4 100644
--- a/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Service.kt
+++ b/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/Service.kt
@@ -133,7 +133,9 @@ data class Service(
       )
     }
 
-    @JvmStatic internal fun fromElements(
+    @JvmStatic
+    @JvmName("fromElements\$wire_schema") // Preserve the Kotlin 2.3 internal JVM name.
+    internal fun fromElements(
       packageName: String?,
       elements: List<ServiceElement>,
     ): List<Service> = elements.map { service ->
@@ -141,7 +143,9 @@ data class Service(
       fromElement(protoType, service)
     }
 
-    @JvmStatic internal fun toElements(services: List<Service>): List<ServiceElement> = services.map { service ->
+    @JvmStatic
+    @JvmName("toElements\$wire_schema") // Preserve the Kotlin 2.3 internal JVM name.
+    internal fun toElements(services: List<Service>): List<ServiceElement> = services.map { service ->
       ServiceElement(
         service.location,
         service.name,