The 18-year-old has passed a medical with the Hammers and have become the club’s eight new signing since manager Sam Allardyce took over.
“I am very excited to come here and play in England, especially for a club like West Ham,” Montenegro told the Hammers’ official website.
“I am hoping to get a chance to show the fans what I can do.
“I have to prove myself but if I get the chance I would love to help the club get promoted this season.
“This is a fantastic opportunity and it would be a dream to help the team get to the Premier League. I am going to work very hard to impress.”