The 29-year-old has signed a two-year deal and becomes Newcastle's second signing of the summer following the arrival of Martin Dubravka.
"I'm so pleased I can join the club. I was always pleasantly surprised when I came to St. James Park to play against Newcastle - there is a great atmosphere and the passion from the fans is a great support to the players," Ki told the club's official website.
"Everyone knows how big this club is so I'm really looking forward to playing with my teammates and for the fans."
Ki was captain of the Korea Republic team at the 2018 FIFA World Cup Russia but missed the 2-0 win over Germany which sent the champions crashing out because of injury."
"I'm pleased to welcome Ki to the club. He is a player with lots of Premier League and international experience and it was an easy decision to bring him here," said Newcastle manager Rafael Benitez.
Ki made 162 appearances for Swansea during his five-year stay, scoring 12 goals, and helped the Welsh team win the club's only major honour by lifting the League Cup in 2012-13.
Sources: AFP, FIFA via Getty Images