West Ham believe teenage Portuguese defender Goncalo Cardoso can make a Premier League impact after he joined from Boavista.
The 18-year-old goes straight into Manuel Pellegrini's first-team squad after agreeing a five-year contract, with the fee involved undisclosed.
The Hammers begin their Premier League season against champions Manchester City on Saturday.
Cardoso said of his move: "I am very excited and it's a dream come true. I always dreamt of coming to England and now I'm here at this big, historic club, so I'm very happy."
West Ham director of football Mario Husillos told the club's website: "He is of course a player for the future, with a lot of potential in front of him, but we feel he is ready to be with the first-team squad from both a physical and technical point of view."