Batshuayi, who joined the Blues in 2016, has spent much of his time at Chelsea out on loan.
He thrived with Borussia Dortmund in 2018, though struggled at Valencia the following season, switching to Crystal Palace on a temporary basis for the back half of 2018-19.
The Belgium forward was a target for Palace in the following transfer window, but remained at Chelsea. He made 24 appearances, though few of them were starts, scoring seven goals.
Batshuayi did return to Selhurst Park last season, though scored just two goals from 18 Premier League appearances for the Eagles.
The 27-year-old had entered the final year of his Chelsea deal, but that has now been renewed, with an immediate season-long loan to Turkish champions Besiktas confirmed on Wednesday.
"Very very happy to join Besiktas," Batshuayi, who received a huge welcome when he landed in Istanbul, tweeted.
"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."
Batshuayi slipped further down the pecking order at Chelsea upon the arrival of his compatriot Romelu Lukaku, who re-joined the European Champions in a reported £97.5million (€115m) deal from Serie A champions Inter last week.
He is the third centre-forward to leave the club this transfer window, with Chelsea having sold Olivier Giroud to Milan, while Tammy Abraham was unveiled as a Roma player on Tuesday.
Thomas Tuchel still has plenty of high-quality attacking options at his disposal, with Lukaku spearheading a forward line that also includes Timo Werner, Kai Havertz and Christian Pulisic, with support provided from midfielders Mason Mount, Hakim Ziyech and Callum Hudson-Odoi.