The Hammers fought off competition from Everton to capture the 22-year-old, who can play at right back or in midfield and has spent the last four seasons in the Leeds first team.
"I'm unbelievably proud and happy to join a club as big as this," said Byram, who grew up in Yorkshire but was born in Essex and favoured a move to London over Merseyside.
"...I just want to get my head down, work hard and hopefully get a chance," he was quoted as saying on the West Ham website (www.whufc.com).
British media put the fee at 3.7 million pounds for a player who played 143 games and scored 10 goals at second-tier Leeds.
"I spoke to the manager (Slaven Bilic) and he is one of the main reasons which attracted me to West Ham. I think he's done unbelievably well with his team so far, so I'm really looking forward to working with him and his staff."
Under Bilic, a former Croatia international who went on to manage his country's national team, West Ham are sixth with 35 points from 22 matches, nine points off top spot and just four points outside the Champions League qualifying places.
Byram, who joined the Elland Road club in 2005 aged 11, can't wait to play in front of his family at the Olympic stadium when West Ham move there next season.
"My uncle and my cousin are big West Ham fans and have bought their tickets for the new stadium," he said.
"My uncle was down here yesterday getting a shirt with my name on the back! It's nice to have them down here and they'll help me settle so I'm looking forward to seeing them."
(Writing by Ken Ferris; editing by John Stonestreet)
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