The Belgian striker's new deal with the Blues is expected to run until 2023.
Chelsea in a statement said;
"Michy Batshuayi has today extended his contract at Chelsea until 2023 and joined Besiktas on loan for the season.
"The Belgian forward heads to Vodafone Park – the venue of our 2019 Super Cup clash with Liverpool - for the remainder of this season, after passing a medical in Istanbul and signing the relevant paperwork. He could make his debut for Besiktas on Saturday."
A loan to Besiktas becomes Batshuayi's fifth loan spell since joining Chelsea in 2016.
Arriving at Chelsea from French Ligue 1 side Marseille, Batshuayi has scored 25 goals in 77 appearances for the European champions.
Batshuayi enjoyed a perfect loan spell in the Bundesliga with Borussia Dortmund in 2018 which many thought he would have made his move permanent but decided to return to London.
Batshuayi has had loan spells with clubs like Borussia Dortmund, Valencia and Crystal Palace, where he scored two goals in 18 league appearances last season.
Feeling excited to have secured a move to his new club, Batshuayi wrote on his Twitter handle.
"Very very happy to join Besiktas an incredible opportunity for me and I'm already looking to play. Thank you to all the fans for the crazy welcome at Istanbul See you at the stadium very soon."
Chelsea will be led in attack by Romelu Lukaku who has rejoined the club from Inter Milan following an impressive stint with the Nerazzurri and is expected to play his first game against Arsenal.
