The 31-year-old England international has joined from Atletico Madrid for £12m, signing a two-and-a-half-year contract.
Trippier reunites with manager Eddie Howe, who managed him earlier on in his career at Burnley.
"I'm delighted to be joining this fantastic club. I really enjoyed my time in Madrid, but when I became aware of interest from Newcastle United, and having worked with Eddie Howe before, I knew this was where I wanted to be," Trippier told the club's official website.
"I'm aware there is a lot of work ahead of us but I know the demands of Premier League well and I know what an amazing club this is with very talented players. I can't wait to get started and I'm excited to step out at St. James' Park as a Newcastle player."
Howe added: "I am really pleased to welcome Kieran to Newcastle United. I have long admired his abilities and have followed his career, so when the opportunity arose to secure him, we didn't hesitate.
"A lot of hard work has gone into planning for the January transfer window long before the month began, which is clear in how quickly we have been able to complete this move."
The former Burnley defender joined Atletico Madrid from Tottenham Hotspurs in the summer of 2019 and made 86 appearances in total for the club, including 15 in La Liga this season.
Trippier helped Atletico win La Liga last year.
Trippier has won 35 caps for the England national team.
