diff --git a/ChangeLog.md b/ChangeLog.md
index 3adfae1c50b..297fefbcf08 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -11,6 +11,9 @@
 - [`KT-12903`](https://youtrack.jetbrains.com/issue/KT-12903) Implement "Inline type alias" refactoring
 - [`KT-12902`](https://youtrack.jetbrains.com/issue/KT-12902) Implement "Introduce type alias" refactoring
 
+###### Issues fixed
+- [`KT-13383`](https://youtrack.jetbrains.com/issue/KT-13383), [`KT-13379`](https://youtrack.jetbrains.com/issue/KT-13379) Override/Implement Members: Do not make return type non-nullable if base return type is explicitly nullable
+
 ## 1.1-M01 (EAP-1)
 
 ### Language features
diff --git a/idea/idea-core/src/org/jetbrains/kotlin/idea/core/overrideImplement/OverrideMemberChooserObject.kt b/idea/idea-core/src/org/jetbrains/kotlin/idea/core/overrideImplement/OverrideMemberChooserObject.kt
index 0fc99583f34..0bb3261288c 100644
--- a/idea/idea-core/src/org/jetbrains/kotlin/idea/core/overrideImplement/OverrideMemberChooserObject.kt
+++ b/idea/idea-core/src/org/jetbrains/kotlin/idea/core/overrideImplement/OverrideMemberChooserObject.kt
@@ -35,6 +35,8 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.findDocComment.findDocComment
 import org.jetbrains.kotlin.renderer.*
 import org.jetbrains.kotlin.resolve.descriptorUtil.setSingleOverridden
+import org.jetbrains.kotlin.types.KotlinType
+import org.jetbrains.kotlin.types.isFlexible
 import org.jetbrains.kotlin.types.typeUtil.makeNotNullable
 
 interface OverrideMemberChooserObject : ClassMember {
@@ -158,7 +160,10 @@ private fun generateConstructorParameter(project: Project, descriptor: PropertyD
 private fun generateFunction(project: Project, descriptor: FunctionDescriptor, bodyType: OverrideMemberChooserObject.BodyType): KtNamedFunction {
     val newDescriptor = object : FunctionDescriptor by descriptor {
         override fun getModality() = Modality.OPEN
-        override fun getReturnType() = descriptor.returnType?.makeNotNullable()
+        override fun getReturnType(): KotlinType? {
+            val originalType = descriptor.returnType ?: return null
+            return if (originalType.isFlexible()) originalType.makeNotNullable() else originalType
+        }
         override fun getOverriddenDescriptors() = listOf(descriptor)
         override fun <R : Any?, D : Any?> accept(visitor: DeclarationDescriptorVisitor<R, D>, data: D) = visitor.visitFunctionDescriptor(this, data)
     }
diff --git a/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/A.java b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/A.java
new file mode 100644
index 00000000000..b4a6d6504b8
--- /dev/null
+++ b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/A.java
@@ -0,0 +1,9 @@
+package foo;
+
+import org.jetbrains.annotations.Nullable;
+
+public class A {
+    public @Nullable String foo(String s) {
+        return null;
+    }
+}
diff --git a/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt
new file mode 100644
index 00000000000..4cfd67ae82c
--- /dev/null
+++ b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt
@@ -0,0 +1,5 @@
+import foo.A
+
+class B : A() {
+    <caret>
+}
diff --git a/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt.after b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt.after
new file mode 100644
index 00000000000..e0e18c01c41
--- /dev/null
+++ b/idea/testData/codeInsight/overrideImplement/nullableJavaType/foo/Impl.kt.after
@@ -0,0 +1,7 @@
+import foo.A
+
+class B : A() {
+    override fun foo(s: String?): String? {
+        <selection><caret>return super.foo(s)</selection>
+    }
+}
diff --git a/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt b/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt
new file mode 100644
index 00000000000..f1a280bc979
--- /dev/null
+++ b/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt
@@ -0,0 +1,7 @@
+interface IBase {
+    fun foo(): Any?
+}
+
+class C : IBase {
+    <caret>
+}
\ No newline at end of file
diff --git a/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt.after b/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt.after
new file mode 100644
index 00000000000..968af4ef22c
--- /dev/null
+++ b/idea/testData/codeInsight/overrideImplement/nullableKotlinType/foo/Impl.kt.after
@@ -0,0 +1,9 @@
+interface IBase {
+    fun foo(): Any?
+}
+
+class C : IBase {
+    override fun foo(): Any? {
+        <selection><caret>TODO("not implemented") //To change body of created functions use File | Settings | File Templates.</selection>
+    }
+}
\ No newline at end of file
diff --git a/idea/tests/org/jetbrains/kotlin/idea/codeInsight/OverrideImplementTest.kt b/idea/tests/org/jetbrains/kotlin/idea/codeInsight/OverrideImplementTest.kt
index 4178cd2ee58..172357f4bdf 100644
--- a/idea/tests/org/jetbrains/kotlin/idea/codeInsight/OverrideImplementTest.kt
+++ b/idea/tests/org/jetbrains/kotlin/idea/codeInsight/OverrideImplementTest.kt
@@ -248,6 +248,14 @@ class OverrideImplementTest : AbstractOverrideImplementTest() {
         doOverrideDirectoryTest("foo")
     }
 
+    fun testNullableJavaType() {
+        doOverrideDirectoryTest("foo")
+    }
+
+    fun testNullableKotlinType() {
+        doOverrideDirectoryTest("foo")
+    }
+
     fun testAbstractAndNonAbstractInheritedFromInterface() {
         doImplementFileTest("getFoo")
     }