Ake, 22, has signed a long-term deal at the Vitality Stadium, returning to the south coast after an impressive loan spell with the Cherries during the first half of the 2016-17 season. Bournemouth have paid a reported fee of £20m for their latest acquisition, eclipsing the £15m they paid Liverpool for Jordon Ibe last summer.
He comes as their third signing of the summer, joining former Blues teammate Asmir Begovic along with Jermain Defoe, who signed a three-year contract with the club on Thursday.
Ake made just 17 appearances for Chelsea after joining the club in 2012, impressing on loan spells away from Stamford Bridge with Reading, Watford and Bournemouth.
While he had to wait until November for his first league start under Eddie Howe last season, the Netherlands international quickly snatched a first-team role, making 12 appearances for the club and scoring some important goals along the way, including late winners against Stoke City and Liverpool.
During the January transfer window, Chelsea activated a recall clause to bring the versatile defender back to west London where he was given just one start in the second half of the season – coming against Watford in April with the Premier League title already wrapped up.
"I had a great time here last season so I'm really happy to be back," Ake told Bournemouth's official website.
"It's an exciting challenge. Chelsea is a great club and I learned so much there but this is a new step and the right place for me to develop even more.
"The fans haven't seen the best of me yet. I always set the bar high so I know there is a lot of things I still have to work on. With the help of the manager here I know I can keep improving and become a better player."
Ake becomes the latest in a growing list of young players to leave Stamford Bridge having failed to break into the first-team.
Bertrand Traore has this week completed a permanent move to side Lyon, admitting his frustration with the role he was given at Chelsea upon signing a deal with the Ligue 1 side. Christian Atsu is another, making his loan stay at Newcastle United permanent following an impressive loan spell at St James' Park last season.
Dominic Solanke meanwhile is poised to leave the club when his contract expires and join Liverpool on 1 July.