The Turkish midfielder has become the Hammers' fifth signing of the summer.
Besiktas star Tore has signed a year-long loan deal with a view to a permanent move to become the Hammers' fifth signing of the summer after midfielders Sofiane Feghouli and Havard Nordtveit and young attackers Toni Martinez and Domingos Quina.
The 24-year-old winger has been an impressive performer over the last three seasons in Turkey, racking up 19 goals and 28 assists in 111 appearances for Besiktas.
In his first two seasons for the Turkish club, Tore played under current Hammers boss Bilic where the two men reportedly forged a close bond on and off the field.
The Turkey international has now reunited with his former coach at the Olympic Stadium and he admitted that was a big influence on his decision to make the move to England.
Tore told the West Ham website: "I am very happy to be at West Ham. I watched many games that they played last season and saw how passionate the fans are. That made me want to come here and I am looking forward to playing in front of them.
"Slaven was also a big factor in me coming here and he is a great manager. He is the main man and knows me very well. I could not have said no to him. He is the best manager I have worked under and he is like a father to the players. You feel like you are family."
Tore is the latest exciting addition for the Hammers as they look to build upon their relatively successful campaign last term in their first season at the Olympic Stadium.
Bilic's side finished seventh in 2015-16, and will be looking to improve upon that placing as they aspire to become regular European challengers.