The 34-year-old England international, who scored 15 league goals last season, has signed a three-year deal.
"It's great to be back," said Defoe, who as a teenager spent the 2000-01 season on loan at Bournemouth from West Ham, scoring in 10 successive matches.
"It was an easy decision, joining a top team with a top manager. The one thing I can guarantee is goals."
The former Tottenham, Portsmouth, West Ham and Charlton player is seventh on the Premier League all-time goals list with 158 in 468 appearances.
Cherries manager Eddie Howe said Defoe's signing is a "huge moment for the club".
Defoe moved to Sunderland in January 2015 after a brief spell at MLS side Toronto and, after saving the club from relegation in the 2015-16 season with 15 goals, signed a contract extension until 2019.
There was a clause in his contract that allowed him to leave on a free transfer if Sunderland were relegated, which he was able to trigger when the Black Cats finished bottom last season.
Defoe made his England debut in 2004 and played at the 2010 World Cup, but did not feature for the national side between 2013 and 2017.
His excellent form last season earned him a recall to Gareth Southgate's side, and he scored his 20th international goal against Lithuania in a World Cup qualifier in March.
Defoe is Bournemouth's second signing of the summer, following the arrival of goalkeeper Asmir Begovic from Chelsea.
"We want to bring players here who can have a big impact on our continued rise and we believe Jermain fits that as a natural, proven goalscorer," added Howe.
The Cherries finished ninth in the Premier League last term - their second season in the top flight.
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Sunderland striker Jermain Defoe has re-signed for former club Bournemouth on a free transfer.