The 24-year-old enjoyed a breakthrough campaign in Portugal last season, keeping 12 clean sheets in 33 league appearances after progressing through Braga's academy following his move from Czech club Pardubice as a teenager.
He is contracted at the club until the summer of 2028, but Newcastle moved decisively to trigger his buyout clause after The Athletic's David Ornstein first reported the club's interest, with Fabrizio Romano subsequently confirming the £25m deal had been sealed.
Hornicek arrives as part of a comprehensive goalkeeping overhaul on Tyneside that has also seen Ewen Jaouen brought in from Reims, with Martin Dubravka departing for Tottenham Hotspur and Aaron Ramsdale returning to Southampton at the end of his loan spell.
Hull City had previously made an unsuccessful attempt to sign the Czech international earlier in the window, but it was Newcastle's move that ultimately proved decisive, with the modern, sweeper-keeper style he has developed at Braga seen as a natural fit for how the club wants to build from the back.
Newcastle United finished 12th in the Premier League last season, and the arrival of a long-term No 1 forms part of a wider summer rebuild that has continued even amid boardroom upheaval, with the club also having sanctioned deals for Bazoumana Toure, Aladji Bamba and Sean Steur.
Hornicek made his senior debut for the Czech Republic having represented his country at every youth level, and is expected to be given every opportunity to establish himself as first-choice between the posts once the new season gets under way.
Standing at 1.97m, Hornicek is viewed internally as the long-term successor to Nick Pope, who has been Newcastle's first-choice goalkeeper in recent seasons, with Jaouen expected to provide competition for the No 2 role behind him rather than challenge immediately for the starting jersey.
The scale of the outlay underlines how seriously the club's recruitment team have tracked the Czech international, having reportedly monitored his progress in Portugal for well over a year before finally moving to complete a deal this week.