<dependency>
    <groupId>com.squareup.retrofit2</groupId>
    <artifactId>retrofit-mock</artifactId>
    <version>${version.retrofit}</version>
    <scope>test</scope>
</dependency>

import retrofit2.mock.Calls;
import static org.mockito.Mockito.when;
import static org.mockito.Mockito.mock;

...

Api api = mock(Api.class); // Mockito mock

...

when(api.doSomething(param)).thenReturn(Calls.response(response));