Besiktas star Gokhan Tore joined West Ham on a one-year loan with a view to a permanent move.
West Ham unveiled Tore as their new transfer on their official site with the Turkish intentional wearing the club shirt.
The Hammers will pay a £2.5 million upfront loan fee and have a £10 million buyout clause option which will remain active until the end of the season.
Tore joining West Ham marks his return to the Premier League after a five-year spell during which time he has picked up Bundesliga, Russian top-tier and Turkish Super Lig experience.
The 24-year-old joined Chelsea as a youth with former academy head Frank Arnesen bringing him in back in 20009 after spotting him in Germany.
Tore was 16 when he joined the Blues and after a two-year spell learning the ropes he was snapped up by HSV which coincided with Arnesen being appointed their sporting director.
Tore then joined Rubin Kazan followed by a move to Besiktas in 2013. Hammers head coach Slaven Bilic took charge of the Black Eagles the following season and coached Tore for two seasons before taking over West Ham.
The Turkish international star won the league title last season and wrapped up his three seasons at the club with 19 goals and 28 assists in 119 games.