Soucek, who joined West Ham on loan in January, has scored three league goals this month as David Moyes's side ensured their Premier League survival with a game to spare.
No financial details were disclosed but British media reported West Ham have agreed a deal worth £19 million pounds with Slavia Prague for the 25-year-old defensive midfielder.
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"I'm delighted that Tomas has signed his permanent contract to secure his long-term future at the club," Moyes told the club's website.
"When I came in, we talked about needing another midfield player, to help Declan Rice and Mark Noble, and I think he's been a really good fit for us.
He's a terrific lad with very good physical capabilities. He has scored some really important goals for us, against Chelsea, Newcastle and Watford
Capped 25 times by his country, Soucek is a two-time Czech first division title winner with Slavia Prague.
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