Soucek joined the Hammers on loan in January but has now agreed a four-year deal.
The 25-year-old has scored three times in the post-lockdown period as the Hammers ensured safety and a ninth successive Premier League season with a game to spare.
“Soucek has been a really good fit for us,” manager David Moyes said.
“When I came in, I talked about needing another midfield player, to help Declan Rice and Mark Noble.
“He’s a terrific lad with very good physical capabilities and has scored some really important goals, against Chelsea, Newcastle and Watford. We’re glad he’s going to be a West Ham player for a good period to come.”
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