The Holland U21 international has signed a five-year deal with an option for a sixth.
“I’m very happy, and I can’t wait to get started,” Summerville told the club's website.
“It's the perfect next step for my career. West Ham United is a huge Club, with lots of ambition and great players, so I’m really excited to be here and to get going.
“I always try to follow my heart, and I had the feeling that this is the best place for me to continue developing as a player, and to push my potential as much as possible.
"There is so much history here, and the team have done so well in the Premier League and in Europe over recent years. I spoke to the Head Coach, and he convinced me that I could make an impact, and help achieve success in the new season.
“I think I'm here to help write more history here, and I'd love to help West Ham get back into Europe. I want to contribute lots of goals and assists, and I'm so excited to meet my teammates and the fans, and get out there now.”