The tall 24-year-old has made 25 senior international appearances.
"I think that he gives us a lot of different qualities, but most importantly he fits the profile of the player we want to bring to the Club," said manager David Moyes. "He is young and hungry but with an established pedigree and proven talent.
"He was captain of Slavia Prague, he's played as a defensive midfielder but he's scored a lot of goals as an attacking midfielder as well.
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"At the moment he ticks a lot of boxes. We need to give him the opportunity and time to settle in but we are confident he will prove to be a strong addition to the squad."
No financial details were given.
The Hammers are 17th in the league, ahead of Bournemouth and Watford only on goal difference.
Moyes took over from the sacked Manuel Pellegrini at the end of December, with West Ham also 17th at the time. (Reporting by Alan Baldwin, editing by Pritha Sarkar)
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