West Ham have announced the signing of midfielder Diego Poyet after agreeing a compensation package with Charlton.
The 19-year-old rejected the offer of a new deal with the Addicks and left the Championship club at the end of his contract on 30 June.
The son of Sunderland manager Gus Poyet has now signed a four-year contract with the Hammers.
"It's massive to come to a Premier League club. It's what I wanted," Diego Poyet told West Ham's official website.
"I wanted a new challenge and soon as I heard there was a chance of stepping up I had to take it.
"Like any young player, the first thing you want to do is get into the first team. I need to show the staff, players and the gaffer what I'm capable of."
Poyet made his Charlton debut in January 2014 and accumulated 23 appearances in the remainder of the campaign before the club's fans voted him player of the season.
"I can't say we are happy with the outcome because our aim from the moment we started negotiating with Diego over a new contract in early 2014 was to keep him at the club in the long term," said Addicks chairman Richard Murray.
Poyet is West Ham's fourth summer signing following the captures of striker Mauro Zarate from Velez Sarsfield, Anderlecht's Senegal international midfielder Cheikhou Kouyate and Ipswich full-back Aaron Cresswell.