The 37-year-old has agreed a one-year deal at St. James' Park and will join the Magpies in preparation for the 2024-25 Premier League season.
Ruddy brings a wealth of experience to the club's goalkeeping unit after clocking up 551 appearances across 13 clubs - including spells in the Premier League with Everton, Norwich City and Wolverhampton Wanderers.
Last season, the former England international stopper made 44 Championship appearances for Birmingham City, missing just two league matches.
John Ruddy said: "This is a massive club and it's a massive opportunity, so I'm really excited to get started.
"It was an opportunity I couldn't turn down. It's a huge, huge club which is only going one way and to be a part of that, however big or small, is going to be very pleasing and very exciting.
"Newcastle United head coach, Eddie Howe, said: "I'm pleased to welcome John to Newcastle United. He has a huge amount of experience and adds a level of support and competition that we need.
"As well as his abilities on the pitch, he has a strong mentality and he is a leader, which will only benefit the group. I'm looking forward to him joining us as we prepare for the season ahead."