Reece Oxford has joined Borussia Monchengladbach on a season-long loan deal.
The West Ham teenager, who can play at centre-half or in midfield, passed a medical at the Bundesliga club on Monday.
The loan will be a second in as many seasons for the 18-year-old, who had a spell at Reading last term.
Hammers boss Slaven Bilic said: "This is a fantastic opportunity for Reece to continue his development and experience top level European football on a regular basis.
"He will play in a league which is very technical, which can only help him immensely, and he will also learn a new culture and gain valuable life experience.
"We believe Reece has a very exciting and successful future here at West Ham, and he now needs regular first team football to ensure his progress continues."
Oxford became West Ham's youngest Premier League player when he started against Arsenal on the opening day of the 2015/16 season.
The England youth international signed a new contract in December.