The Spaniard is heading back to his former club for a reported £53m after both clubs made the arrangement public.
It brings to an end a troubled spell for the marksman who went AWOL this summer. He'd been told by boss Antonio Conte that he was surplus to requirements at the club.
Costa, who had two years remaining on his Chelsea contract, jetted to Brazil and defied attempts by the Premier League champions to force him to return.
