The actress - who has played lead character Dr Addison Montgomery in the Grey's Anatomy spinoff for five seasons - was previously rumoured to have signed up for just one more season, and has now confirmed her future departure.
Walsh told talk show host Bethenny Frankel that she was thankful for her time on both Shonda Rhimes dramas, and that she is looking forward to the show's climax.
"This is my last season for Private Practice," she said. "It's been an incredible journey, an amazing ride. I'm hugely, hugely grateful.
"It's been a long time being Addison. I feel incredibly grateful to Shonda Rhimes and all the fans that have been there from Day 1 to now. I'm really excited about the last season."
Private Practice was ordered for a sixth run of a reduced 13 episodes by ABC earlier this year.
Walsh's co-star Tim Daly will not be returning for the sixth season, as his contract was not renewed.
Watch Kate Walsh appear on Wednesday's episode of Bethany below: