Although Newcastle had already brought in 20-year-old Ewen Jaouen from Reims this summer, the club had also been in the market for a more established number one.
Hornicek, 24, flew out to Newcastle's training camp in Spain at the weekend to finalise his move and sign a five-year contract after the club activated his release clause, becoming Newcastle's fifth summer signing following the arrivals of Jaouen, Bazoumana Toure, Sean Steur and Aladji Bamba.
"The last few days have been an emotional rollercoaster from when I first heard about the move and now I'm so happy to be here," the Czech Republic international said.
"It's a brilliant opportunity in a new league, which I'm very excited about. From the very first moment I arrived here, I received a fantastic welcome and really felt the warmth of the club.
"Everyone has been brilliant in helping me settle in. I know Newcastle is a huge football city, with 52,000 fans at St James' Park creating an incredible atmosphere and pushing the team every game. I can't wait to play in front of them."
Newcastle United had tracked Hornicek's progress as he kept seven clean sheets during Braga's run to the Europa League semi-finals last season, the most of any goalkeeper in the competition, while also helping the Portuguese club finish fourth domestically and recording 24 clean sheets across all competitions.
As well as attempting to keep the ball out of the net, Opta data revealed Hornicek ranked second for successful passes (846) and fourth for passing accuracy (75%) among goalkeepers in Portugal's top flight last season. He had turned down a move to newly promoted Hull City earlier in the summer despite the Championship club activating his release clause, choosing to wait for interest from the Premier League instead.
Newcastle sporting director Ross Wilson said the 24-year-old's "progress in recent years has been remarkable."
"He has firmly established himself as one of Europe's top young goalkeepers," Wilson said. "We knew there would be no shortage of clubs interested in acquiring Lukas this summer after everything he achieved last season, but it quickly became evident how keen he was to be a part of what we are building here."
Hornicek joined Braga permanently in 2020 after an initial loan spell from Czech club Pardubice and has since become one of the most reliable goalkeepers in Portugal's top flight, standing at 1.97m tall.