Jack Sullivan, son of West Ham co-owner David Sullivan, has confirmed the capture of Diego Poyet on Twitter.
Poyet, 19, had been expected to join the Hammers after goalkeeper Adrian tweeted a picture of himself, forward Mauro Zarate and Poyet in France for the club's pre-season training camp.
An industrious central midfielder, Poyet is the son of Sunderland boss and former Chelsea midfielder, Gus.
I can now confirm POYET is OFFICIALLY a WEST HAM PLAYER !!! Go to official club website for more info later pic.twitter.com/up3ZqqxPPL— Jack Sullivan (@jsullivanwhu) July 8, 2014
The Spaniard only made his first team debut for Charlton Athletic last season before informing the club he would not return for pre-season training following the expiration of his contract.