Former Chelsea player spent six months on loan at the Cherries last season, and has now signed a long-term deal at the club, joining shortly after the addition of veteran striker Jermain Defoe.
Ake was delighted to complete the move to Bournemouth and eager to continue improving his game under the guidance of manager Eddie Howe.
“I had a great time here last season so I’m really happy to be back,” said Ake, according to Sky Sports.
“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.”