diff --git a/plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrSerializableProperties.kt b/plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrSerializableProperties.kt
index 1136aedbb25..7f954f05c59 100644
--- a/plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrSerializableProperties.kt
+++ b/plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrSerializableProperties.kt
@@ -10,27 +10,25 @@ import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.declarations.IrClass
 import org.jetbrains.kotlin.ir.declarations.IrProperty
 import org.jetbrains.kotlin.ir.types.IrSimpleType
-import org.jetbrains.kotlin.ir.util.hasAnnotation
-import org.jetbrains.kotlin.ir.util.hasDefaultValue
-import org.jetbrains.kotlin.ir.util.primaryConstructor
-import org.jetbrains.kotlin.ir.util.properties
+import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.serialization.deserialization.descriptors.DeserializedPropertyDescriptor
 import org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationDescriptorSerializerPlugin
-import org.jetbrains.kotlinx.serialization.compiler.extensions.SerializationPluginContext
 import org.jetbrains.kotlinx.serialization.compiler.resolve.*
 
 class IrSerializableProperty(
     val ir: IrProperty,
     override val isConstructorParameterWithDefault: Boolean,
     hasBackingField: Boolean,
-    declaresDefaultValue: Boolean
+    declaresDefaultValue: Boolean,
+    val type: IrSimpleType
 ) : ISerializableProperty {
     override val name = ir.annotations.serialNameValue ?: ir.name.asString()
     override val originalDescriptorName: Name = ir.name
-    val type = ir.getter!!.returnType as IrSimpleType
     val genericIndex = type.genericIndex
-    fun serializableWith(ctx: SerializationBaseContext) = ir.annotations.serializableWith() ?: analyzeSpecialSerializers(ctx, ir.annotations)
+    fun serializableWith(ctx: SerializationBaseContext) =
+        ir.annotations.serializableWith() ?: analyzeSpecialSerializers(ctx, ir.annotations)
+
     override val optional = !ir.annotations.hasAnnotation(SerializationAnnotations.requiredAnnotationFqName) && declaresDefaultValue
     override val transient = ir.annotations.hasAnnotation(SerializationAnnotations.serialTransientFqName) || !hasBackingField
 }
@@ -42,10 +40,27 @@ class IrSerializableProperties(
     override val serializableStandaloneProperties: List<IrSerializableProperty>
 ) : ISerializableProperties<IrSerializableProperty>
 
+/**
+ * typeReplacement should be populated from FakeOverrides and is used when we want to determine the type for property
+ * accounting for generic substitutions performed in subclasses:
+ *
+ * ```
+ *    @Serializable
+ *    sealed class TypedSealedClass<T>(val a: T) {
+ *        @Serializable
+ *        data class Child(val y: Int) : TypedSealedClass<String>("10")
+ *     }
+ * ```
+ * In this case, serializableProperties for TypedSealedClass is a listOf(IrSerProp(val a: T)),
+ * but for Child is a listOf(IrSerProp(val a: String), IrSerProp(val y: Int)).
+ *
+ * Using this approach, we can correctly deserialize parent's properties in Child.Companion.deserialize()
+ */
 @OptIn(ObsoleteDescriptorBasedAPI::class)
 internal fun serializablePropertiesForIrBackend(
     irClass: IrClass,
-    serializationDescriptorSerializer: SerializationDescriptorSerializerPlugin? = null
+    serializationDescriptorSerializer: SerializationDescriptorSerializerPlugin? = null,
+    typeReplacement: Map<IrProperty, IrSimpleType>? = null
 ): IrSerializableProperties {
     val properties = irClass.properties.toList()
     val primaryConstructorParams = irClass.primaryConstructor?.valueParameters.orEmpty()
@@ -68,14 +83,17 @@ internal fun serializablePropertiesForIrBackend(
         .map {
             val isConstructorParameterWithDefault = primaryParamsAsProps[it] ?: false
             // FIXME: workaround because IrLazyProperty doesn't deserialize information about backing fields. Fallback to descriptor won't work with FIR.
-            val isPropertyFromAnotherModuleDeclaresDefaultValue = it.descriptor is DeserializedPropertyDescriptor &&  it.descriptor.declaresDefaultValue()
-            val isPropertyWithBackingFieldFromAnotherModule = it.descriptor is DeserializedPropertyDescriptor && (it.descriptor.backingField != null || isPropertyFromAnotherModuleDeclaresDefaultValue)
+            val isPropertyFromAnotherModuleDeclaresDefaultValue =
+                it.descriptor is DeserializedPropertyDescriptor && it.descriptor.declaresDefaultValue()
+            val isPropertyWithBackingFieldFromAnotherModule =
+                it.descriptor is DeserializedPropertyDescriptor && (it.descriptor.backingField != null || isPropertyFromAnotherModuleDeclaresDefaultValue)
             IrSerializableProperty(
                 it,
                 isConstructorParameterWithDefault,
                 it.backingField != null || isPropertyWithBackingFieldFromAnotherModule,
                 it.backingField?.initializer.let { init -> init != null && !init.expression.isInitializePropertyFromParameter() } || isConstructorParameterWithDefault
-                        || isPropertyFromAnotherModuleDeclaresDefaultValue
+                        || isPropertyFromAnotherModuleDeclaresDefaultValue,
+                typeReplacement?.get(it) ?: it.getter!!.returnType as IrSimpleType
             )
         }
         .filterNot { it.transient }
@@ -83,13 +101,22 @@ internal fun serializablePropertiesForIrBackend(
 
     var serializableProps = run {
         val supers = irClass.getSuperClassNotAny()
-        if (supers == null || !supers.isInternalSerializable)
+        if (supers == null || !supers.isInternalSerializable) {
             primaryCtorSerializableProps + bodySerializableProps
-        else
+        } else {
+            val originalToTypeFromFO = typeReplacement ?: buildMap<IrProperty, IrSimpleType> {
+                irClass.properties.filter { it.isFakeOverride }.forEach { prop ->
+                    val orig = prop.resolveFakeOverride()
+                    val type = prop.getter?.returnType as? IrSimpleType
+                    if (orig != null && type != null) put(orig, type)
+                }
+            }
             serializablePropertiesForIrBackend(
                 supers,
-                serializationDescriptorSerializer
+                serializationDescriptorSerializer,
+                originalToTypeFromFO
             ).serializableProperties + primaryCtorSerializableProps + bodySerializableProps
+        }
     }
 
     // FIXME: since descriptor from FIR does not have classProto in it(?), this line won't do anything
@@ -99,4 +126,4 @@ internal fun serializablePropertiesForIrBackend(
         irClass.isInternallySerializableEnum() || primaryConstructorParams.size == primaryParamsAsProps.size
 
     return IrSerializableProperties(serializableProps, isExternallySerializable, primaryCtorSerializableProps, bodySerializableProps)
-}
\ No newline at end of file
+}
diff --git a/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassMultiple.kt b/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassMultiple.kt
new file mode 100644
index 00000000000..c46c0987099
--- /dev/null
+++ b/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassMultiple.kt
@@ -0,0 +1,56 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+@Serializable
+abstract class Top<T: Any> {
+    var top: T? = null
+}
+
+@Serializable
+open class Intermediate<V>: Top<List<V>>() {
+    var inter: V? = null
+    override fun toString(): String {
+        return "Intermediate($top, $inter)"
+    }
+}
+
+@Serializable
+open class Bottom: Intermediate<String>() {
+    var bot: String? = null
+    override fun toString(): String {
+        return "Bottom($top, $inter, $bot)"
+    }
+}
+
+@Serializable
+class Bottom2: Bottom() {
+    override fun toString(): String {
+        return "Bottom2($top, $inter, $bot)"
+    }
+}
+
+@Serializable
+data class Full(
+    val b: Bottom2,
+    val i: Intermediate<String>
+)
+
+fun box(): String {
+    val j = Json { ignoreUnknownKeys = true }
+    val b = Bottom2().apply {
+        top = listOf("a", "b")
+        inter = "v"
+        bot = "bot"
+    }
+    val f = Full(b, b)
+    val encoded = j.encodeToString(f)
+    if (encoded != """{"b":{"top":["a","b"],"inter":"v","bot":"bot"},"i":{"top":["a","b"],"inter":"v"}}""") return "Encoded: $encoded"
+
+    val decoded = j.decodeFromString<Full>(encoded)
+    if (decoded.toString() != "Full(b=Bottom2([a, b], v, bot), i=Intermediate([a, b], v))") return "Decoded: $decoded"
+    return "OK"
+}
diff --git a/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassSimple.kt b/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassSimple.kt
new file mode 100644
index 00000000000..766e70d4cf8
--- /dev/null
+++ b/plugins/kotlinx-serialization/testData/boxIr/genericBaseClassSimple.kt
@@ -0,0 +1,46 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+
+// From #1264
+@Serializable
+sealed class TypedSealedClass<T>(val a: T) {
+    @Serializable
+    class Child(val y: Int) : TypedSealedClass<String>("10") {
+        override fun toString(): String = "Child($a, $y)"
+    }
+}
+
+// From #KT-43910
+@Serializable
+open class ValidatableValue<T : Any, V: Any>(
+    var value: T? = null,
+    var error: V? = null,
+)
+
+@Serializable
+class Email<T: Any> : ValidatableValue<String, T>() { // Note this is a different T
+    override fun toString(): String {
+        return "Email($value, $error)"
+    }
+}
+
+fun box(): String {
+    val encodedChild = """{"a":"11","y":42}"""
+    val decodedChild = Json.decodeFromString<TypedSealedClass.Child>(encodedChild)
+    if (decodedChild.toString() != "Child(11, 42)") return "DecodedChild: $decodedChild"
+    Json.encodeToString(decodedChild)?.let { if (it != encodedChild) return "EncodedChild: $it" }
+
+    val email = Email<Int>().apply {
+        value = "foo"
+        error = 1
+    }
+    val encodedEmail = Json.encodeToString(email)
+    if (encodedEmail != """{"value":"foo","error":1}""") return "EncodedEmail: $encodedEmail"
+    val decodedEmail = Json.decodeFromString<Email<Int>>(encodedEmail)
+    if (decodedEmail.toString() != "Email(foo, 1)") return "DecodedEmail: $decodedEmail"
+    return "OK"
+}
\ No newline at end of file
diff --git a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java
index fe56f076125..456ad14c8e8 100644
--- a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java
+++ b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java
@@ -51,6 +51,18 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
         }
 
+        @Test
+        @TestMetadata("genericBaseClassMultiple.kt")
+        public void testGenericBaseClassMultiple() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/genericBaseClassMultiple.kt");
+        }
+
+        @Test
+        @TestMetadata("genericBaseClassSimple.kt")
+        public void testGenericBaseClassSimple() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/genericBaseClassSimple.kt");
+        }
+
         @Test
         @TestMetadata("generics.kt")
         public void testGenerics() throws Exception {
diff --git a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java
index 2425cdec948..fd346155a19 100644
--- a/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java
+++ b/plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java
@@ -49,6 +49,18 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
     }
 
+    @Test
+    @TestMetadata("genericBaseClassMultiple.kt")
+    public void testGenericBaseClassMultiple() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/genericBaseClassMultiple.kt");
+    }
+
+    @Test
+    @TestMetadata("genericBaseClassSimple.kt")
+    public void testGenericBaseClassSimple() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/genericBaseClassSimple.kt");
+    }
+
     @Test
     @TestMetadata("generics.kt")
     public void testGenerics() throws Exception {