Demba Ba has completed his transfer from Chelsea to Besiktas, the Turkish club have announced on their official website.
Besiktas claim the deal for the 29-year-old Senegal international is worth 6 million euros (4.7m pounds).
Ba arrived in Istanbul on Wednesday to undergo a medical and pose with the No. 9 jersey he will wear next season.
"I've had a lot of positive feedback about Besiktas and Istanbul from my Senegalese friends in Turkey," the striker said on Thursday.
"To play in a Muslim country and in a new stadium is definitely very exciting. I know what my coach Slaven Bilic expects from me, and I am open to all sorts of competition.
"Since Besiktas fans like hard-working and spirited players, I will give everything I've got in order not to disappoint them."
Ba joined West Ham from Bundesliga club Hoffenheim in 2011 and moved on to Newcastle in the same year.
After scoring 29 goals in 54 appearances for the Magpies he moved to Chelsea, but made only 23 starts scoring 14 goals.
Chelsea confirmed the deal on their official website, stating: "Everybody at Chelsea Football Club would like to thank Demba for his service and wish him all the best for the future."
Besiktas president Fikret Orman spoke of his delight with the club's new acquisition this week.
"Ba is a player we all believe in, we put in a lot of time and effort to complete the transfer to the best of our interests," Orman said. "The transfer is within our budget and we are delighted to have completed the move."