The England full-back returns to the Premier League after a successful two-and-a-half year spell in Spain.
The defender has signed a two-and-a-half-year deal and becomes Newcastle's first signing since being taken over by a consortium led by Saudi Arabia's Public Investment Fund in October.
"I'm delighted to be joining this fantastic club," Trippier told the club's official website. "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.
"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."
I'm so happy to be joining Newcastle. Everyone knows the fans are unbelievably passionate and I will give my absolute all for them. I can't wait to get to work 💪🏼— Kieran Trippier (@trippier2) January 7, 2022
Thank you to everyone at @nufc & the fans for the very warm welcome I have received so far 🖤🤍 pic.twitter.com/6d0raRrpnv
Trippier, 31, made 86 appearances for Atletico in all competitions after joining from Tottenham in 2019, and helped them win the LaLiga title last season.
ESPN reported in December that Atletico were anticipating a bid for Trippier from Newcastle when the January transfer window opened.
"I am really pleased to welcome Kieran to Newcastle United," Newcastle boss Howe said. "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. I would like to thank everyone involved, particularly our owners, and I'm very much looking forward to working with Kieran again on the training pitch."
The club and coach Diego Simeone were keen to keep Trippier -- who had a contract at the Wanda Metroplitano until 2023 -- and considered him a key member of the team.
However there was an acceptance that the defender favoured a return to England, having previously expressed an interest in joining Manchester United last summer.
"He's extraordinary. Since he arrived, he's always given us a lot of great things," Simeone said last week. "We want him to stay, but nowadays you can't hold anyone prisoner."
Trippier played in Atletico's 2-0 LaLiga win over Rayo Vallecano on Sunday but then missed training on Tuesday as the move neared completion.
Sources told ESPN that Atletico had already begun looking at possible replacements for Trippier and were prioritising the signing of a right-back who could also play in central defence.
The club are expected to invest heavily during the January window in a bid to avoid relegation from the Premier League.