The 19-year-old midfielder spent most of last season on loan at Yeovil making 30 appearances and scoring once in helping them to avoid relegation.
Many had seen Cohen as one of the next Academy stars to cement a place in Alan Pardew's squad but following last season's successes his place could not be guaranteed and Chris decided he wanted to be playing regular first-team football.
Speaking to Yeovil's official website he expressed his sadness at leaving the Hammers, “I had a good loan spell here and it was the best deal for me to come back.
”I am however a little sad to be leaving West Ham. There's an element of that when you leave any club - especially a club that you supported as a boy.
”But sometimes you just have to move on and I'm going to try my best for Yeovil Town; that's all that matters now.”