West Ham have signed Swansea midfielder Flynn Downes on a five-year contract, in a deal understood to be in the region of £12m.
However, he excelled last term and attracted the attention of a number of clubs.
Downes will wear the number 12 shirt and said it is "surreal" to be joining his boyhood club.
"I feel like I'm dreaming," said Downes, who has the option for another year in his new deal at London Stadium.
"It's been my dream since I was a little boy to join this club - it's an absolute privilege.