The 18-year-old Spain under-19 international, who is yet to play for Valencia's first team, has signed a three-year contract which starts in July but will begin training with West Ham immediately.
"For me, it's a great opportunity to be here and I hope to make the most of being at such a big club," Martinez said.
"I joined West Ham because of the progress that the club is making. The decision to come was made because it is a brilliant opportunity to join an interesting Premier League club."
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West Ham, sixth in the table and chasing a place in Europe, will move to the 60,000-seater Olympic Stadium next season.
(Reporting by Martyn Herman; Editing by Ken Ferris)
