The former Celtic player, 24, made 38 appearances for the south Wales side in all competitions last season but has fallen down the pecking order this term and has been allowed to leave.
Sunderland confirmed on their website: "Sunderland have signed midfielder Ki Sung-yueng from Swansea City on a season-long loan. The 24-year-old South Korean international is the club's 11th summer signing. He is not eligible for today's [Saturday's] game against Crystal Palace."
Ki started his career at FC Seoul and moved to Europe when Celtic gave him his big chance in January 2010.
He won the Scottish Cup and league during his time at Parkhead, scoring 11 goals in 85 games and prompting the Swans to invest a then-club-record £5.5million to secure his services in August last year.
However, just 12 months into a three-year deal, he finds himself surplus to requirements after Swansea boss Michael Laudrup this summer added the likes of Jonjo Shelvey and Jose Canas to his central midfield ranks.
Ki will link up with international colleague Ji Dong-won at the Stadium of Light.