diff --git a/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/metadata.kt b/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/metadata.kt
index 4ccefbd..6d87af5 100644
--- a/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/metadata.kt
+++ b/kotlin/codegen/src/main/java/com/squareup/moshi/kotlin/codegen/metadata.kt
@@ -57,6 +57,7 @@ import me.eugeniomarletti.kotlin.metadata.kotlinMetadata
 import me.eugeniomarletti.kotlin.metadata.modality
 import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.Class
 import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.Modality.ABSTRACT
+import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.Modality.SEALED
 import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.Type
 import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.TypeParameter
 import me.eugeniomarletti.kotlin.metadata.shadow.metadata.ProtoBuf.TypeParameter.Variance
@@ -277,6 +278,11 @@ internal fun targetType(messager: Messager,
           ERROR, "@JsonClass can't be applied to $element: must not be an inner class", element)
       return null
     }
+    proto.modality == SEALED -> {
+      messager.printMessage(
+          ERROR, "@JsonClass can't be applied to $element: must not be sealed", element)
+      return null
+    }
     proto.modality == ABSTRACT -> {
       messager.printMessage(
           ERROR, "@JsonClass can't be applied to $element: must not be abstract", element)
diff --git a/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt b/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
index dbaacce..99896ea 100644
--- a/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
+++ b/kotlin/codegen/src/test/java/com/squareup/moshi/kotlin/codegen/JsonClassCodegenProcessorTest.kt
@@ -118,6 +118,23 @@ class JsonClassCodegenProcessorTest {
         "error: @JsonClass can't be applied to AbstractClass: must not be abstract")
   }
 
+  @Test fun sealedClassesNotSupported() {
+    val call = KotlinCompilerCall(temporaryFolder.root)
+    call.inheritClasspath = true
+    call.addService(Processor::class, JsonClassCodegenProcessor::class)
+    call.addKt("source.kt", """
+        |import com.squareup.moshi.JsonClass
+        |
+        |@JsonClass(generateAdapter = true)
+        |sealed class SealedClass(val a: Int)
+        |""".trimMargin())
+
+    val result = call.execute()
+    assertThat(result.exitCode).isEqualTo(ExitCode.COMPILATION_ERROR)
+    assertThat(result.systemErr).contains(
+        "error: @JsonClass can't be applied to SealedClass: must not be sealed")
+  }
+
   @Test fun innerClassesNotSupported() {
     val call = KotlinCompilerCall(temporaryFolder.root)
     call.inheritClasspath = true
diff --git a/kotlin/reflect/src/main/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapter.kt b/kotlin/reflect/src/main/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapter.kt
index 2d0d7a9..0aafe56 100644
--- a/kotlin/reflect/src/main/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapter.kt
+++ b/kotlin/reflect/src/main/java/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapter.kt
@@ -202,6 +202,9 @@ class KotlinJsonAdapterFactory : JsonAdapter.Factory {
     require(rawTypeKotlin.objectInstance == null) {
       "Cannot serialize object declaration ${rawType.name}"
     }
+    require(!rawTypeKotlin.isSealed) {
+      "Cannot reflectively serialize sealed class ${rawType.name}. Please register an adapter."
+    }
 
     val constructor = rawTypeKotlin.primaryConstructor ?: return null
     val parametersByName = constructor.parameters.associateBy { it.name }
diff --git a/kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt b/kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt
index 18ce283..953d71f 100644
--- a/kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt
+++ b/kotlin/tests/src/test/kotlin/com/squareup/moshi/kotlin/reflect/KotlinJsonAdapterTest.kt
@@ -1113,6 +1113,21 @@ class KotlinJsonAdapterTest {
   @JvmSuppressWildcards(suppress = false)
   data class MapOfStringToClassCodegen(val map: Map<String, ConstructorParameters> = mapOf())
 
+  @Test fun sealedClassesAreRejected() {
+    val moshi = Moshi.Builder()
+        .add(KotlinJsonAdapterFactory())
+        .build()
+
+    try {
+      moshi.adapter<SealedClass>()
+      fail()
+    } catch (e: IllegalArgumentException) {
+      assertThat(e).hasMessageContaining("Cannot reflectively serialize sealed class")
+    }
+  }
+
+  sealed class SealedClass
+
   private fun <T> mapWildcardsParameterizedTest(type: Class<T>, json: String, value: T) {
     // Ensure the map was created with the expected wildcards of a Kotlin map.
     val fieldType = type.getDeclaredField("map").genericType