The 23-year-old has been a target of Hammers boss David Moyes since the start of the window and joins on a season-long loan with an option to buy of £12million with £7million in add-ons.
Should Kral make a certain number of starts for West Ham during his time on loan over the season then that option becomes and obligation for the east Londoners.
Kral, formerly of Slavia Prague, joins Czech Republic teammates Tomas Soucek and Vladimir Coufal at the London Stadium and will meet the rest of the squad following the international break. Kral was named in the Czech squad for this current break.
Soucek and England international Declan Rice were outstanding in the heart of midfield last season but Moyes has been keen to add depth to that position with the added burden of the Europa League being placed on his squad this season.
Kral is the second deadline day arrival at West Ham with Croatia international, Nikola Vlasic, having signed from CSKA Moscow in a deal worth £25.7million with a further £8million in add-ons. Kurt Zouma joined in a £29.8million deal from Chelsea last week while goalkeeper Alphonse Areola was signed on loan from Paris Saint-Germain before the start of the season.
“I am delighted to welcome Alex to West Ham United,” Moyes said. “He will provide good competition in the centre of midfield. He is another young, talented player who we feel has a big potential and will be capable of adapting to the demands of the Premier League.
“We’ve brought him in on loan initially and, hopefully, over the course of the season he will make a valuable contribution, particularly with our busier schedule of fixtures.
“Alex is another established international player with the Czech Republic, and we hope he has similar characteristics and hunger to that of Tomas and Vladimir.”
West Ham could still add a young player for the future before the close of the deadline.