diff --git a/runtime/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt b/runtime/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt
index e166a09735..639b3b92e8 100644
--- a/runtime/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt
+++ b/runtime/commonMain/src/kotlinx/serialization/json/internal/Polymorphic.kt
@@ -53,8 +53,8 @@ internal fun <T> JsonInput.decodeSerializableValuePolymorphic(deserializer: Dese
     }
 
     val jsonTree = cast<JsonObject>(decodeJson())
-    val type = jsonTree.getValue(json.configuration.classDiscriminator).content
-    (jsonTree.content as MutableMap).remove(json.configuration.classDiscriminator)
+    val discriminator = json.configuration.classDiscriminator
+    val type = jsonTree.getValue(discriminator).content
     val actualSerializer = deserializer.findPolymorphicSerializer(this, type).cast<T>()
-    return json.readJson(jsonTree, actualSerializer)
+    return json.readPolymorphicJson(discriminator, jsonTree, actualSerializer)
 }
diff --git a/runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonInput.kt b/runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonInput.kt
index 76acb039c1..6572eb01c7 100644
--- a/runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonInput.kt
+++ b/runtime/commonMain/src/kotlinx/serialization/json/internal/TreeJsonInput.kt
@@ -21,6 +21,14 @@ internal fun <T> Json.readJson(element: JsonElement, deserializer: Deserializati
     return input.decode(deserializer)
 }
 
+internal fun <T> Json.readPolymorphicJson(
+    discriminator: String,
+    element: JsonObject,
+    deserializer: DeserializationStrategy<T>
+): T {
+    return JsonTreeInput(this, element, discriminator, deserializer.descriptor).decode(deserializer)
+}
+
 private sealed class AbstractJsonTreeInput(
     override val json: Json,
     open val value: JsonElement
@@ -140,7 +148,12 @@ private class JsonPrimitiveInput(json: Json, override val value: JsonPrimitive)
     }
 }
 
-private open class JsonTreeInput(json: Json, override val value: JsonObject) : AbstractJsonTreeInput(json, value) {
+private open class JsonTreeInput(
+    json: Json,
+    override val value: JsonObject,
+    private val polyDiscriminator: String? = null,
+    private val polyDescriptor: SerialDescriptor? = null
+) : AbstractJsonTreeInput(json, value) {
     private var position = 0
 
     override fun decodeElementIndex(descriptor: SerialDescriptor): Int {
@@ -155,12 +168,23 @@ private open class JsonTreeInput(json: Json, override val value: JsonObject) : A
 
     override fun currentElement(tag: String): JsonElement = value.getValue(tag)
 
+    override fun beginStructure(descriptor: SerialDescriptor): CompositeDecoder {
+        /*
+         * For polymorphic serialization we'd like to avoid excessive decoder creating in
+         * beginStructure to properly preserve 'polyDiscriminator' field and filter it out.
+         */
+        if (descriptor === polyDescriptor) return this
+        return super.beginStructure(descriptor)
+    }
+
     override fun endStructure(descriptor: SerialDescriptor) {
         if (configuration.ignoreUnknownKeys || descriptor.kind is PolymorphicKind) return
         // Validate keys
         val names = descriptor.cachedSerialNames()
         for (key in value.keys) {
-            if (key !in names) throw UnknownKeyException(key, value.toString())
+            if (key !in names && key != polyDiscriminator) {
+                throw UnknownKeyException(key, value.toString())
+            }
         }
     }
 }
diff --git a/runtime/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt b/runtime/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
index 8e233fff11..8cb51e4d15 100644
--- a/runtime/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
+++ b/runtime/commonTest/src/kotlinx/serialization/features/PolymorphismTest.kt
@@ -49,7 +49,7 @@ class PolymorphismTest : JsonTestBase() {
     object PolyDefaultSerializer : JsonTransformingSerializer<PolyDefault>(PolyDefault.serializer(), "foo") {
         override fun readTransform(element: JsonElement): JsonElement {
             return buildJson {
-                add("json", element)
+                add("json", JsonObject(element.jsonObject.filterKeys { it != "type" }))
                 add("id", 42)
             }
         }
diff --git a/runtime/commonTest/src/kotlinx/serialization/features/SealedPolymorphismTest.kt b/runtime/commonTest/src/kotlinx/serialization/features/SealedPolymorphismTest.kt
index f264b1c781..5531070aa0 100644
--- a/runtime/commonTest/src/kotlinx/serialization/features/SealedPolymorphismTest.kt
+++ b/runtime/commonTest/src/kotlinx/serialization/features/SealedPolymorphismTest.kt
@@ -10,21 +10,24 @@ import kotlinx.serialization.modules.SerializersModule
 import kotlinx.serialization.test.assertStringFormAndRestored
 import kotlin.test.Test
 
-@Serializable
-data class FooHolder(
-    val someMetadata: Int,
-    val payload: List<@Polymorphic Foo>
-)
-
-@Serializable
-sealed class Foo {
-    @Serializable
-    data class Bar(val bar: Int) : Foo()
+class SealedPolymorphismTest {
+
     @Serializable
-    data class Baz(val baz: String) : Foo()
-}
+    data class FooHolder(
+        val someMetadata: Int,
+        val payload: List<@Polymorphic Foo>
+    )
 
-class SealedPolymorphismTest {
+    @Serializable
+    @SerialName("Foo")
+    sealed class Foo {
+        @Serializable
+        @SerialName("Bar")
+        data class Bar(val bar: Int) : Foo()
+        @Serializable
+        @SerialName("Baz")
+        data class Baz(val baz: String) : Foo()
+    }
 
     val sealedModule = SerializersModule {
         polymorphic(Foo::class) {
@@ -39,8 +42,8 @@ class SealedPolymorphismTest {
     fun testSaveSealedClassesList() {
         assertStringFormAndRestored(
             """{"someMetadata":42,"payload":[
-            |{"type":"kotlinx.serialization.features.Foo.Bar","bar":1},
-            |{"type":"kotlinx.serialization.features.Foo.Baz","baz":"2"}]}""".trimMargin().replace("\n", ""),
+            |{"type":"Bar","bar":1},
+            |{"type":"Baz","baz":"2"}]}""".trimMargin().replace("\n", ""),
             FooHolder(42, listOf(Foo.Bar(1), Foo.Baz("2"))),
             FooHolder.serializer(),
             json,
@@ -51,7 +54,7 @@ class SealedPolymorphismTest {
     @Test
     fun testCanSerializeSealedClassPolymorphicallyOnTopLevel() {
         assertStringFormAndRestored(
-            """{"type":"kotlinx.serialization.features.Foo.Bar","bar":1}""",
+            """{"type":"Bar","bar":1}""",
             Foo.Bar(1),
             PolymorphicSerializer(Foo::class),
             json
diff --git a/runtime/commonTest/src/kotlinx/serialization/json/polymorphic/JsonDeserializePolymorphicTwiceTest.kt b/runtime/commonTest/src/kotlinx/serialization/json/polymorphic/JsonDeserializePolymorphicTwiceTest.kt
new file mode 100644
index 0000000000..298fd91623
--- /dev/null
+++ b/runtime/commonTest/src/kotlinx/serialization/json/polymorphic/JsonDeserializePolymorphicTwiceTest.kt
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+package kotlinx.serialization.json.polymorphic
+
+import kotlinx.serialization.Serializable
+import kotlinx.serialization.json.*
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class JsonDeserializePolymorphicTwiceTest {
+
+    @Serializable
+    sealed class Foo {
+        @Serializable
+        data class Bar(val a: Int) : Foo()
+    }
+
+    @Test
+    fun testDeserializeTwice() { // #812
+        val json = Json.toJson(Foo.serializer(), Foo.Bar(1))
+        assertEquals(Foo.Bar(1), Json.fromJson(Foo.serializer(), json))
+        assertEquals(Foo.Bar(1), Json.fromJson(Foo.serializer(), json))
+    }
+}
\ No newline at end of file