Maynard has been a revelation at Bristol City scoring 46 goals in 130 games and he has been chased by a number of other Championship sides in recent months.
Maynard admits that the lure of Premiership football next season was behind his switch to the Boleyn Ground.
“I’m ecstatic. It’s been a long day but I’m delighted to put pen to paper and can’t wait to get going, Maynard told the official site. West Ham are a massive club. They’ve got everything geared to get to the Premier League and the Premier League is where I want to be.
“I’m a quick player who likes to get in behind and thrives off chances. I’ll keep plugging away all game and as I’m not happy unless I’m scoring goals.”