TVGuide.com said Walsh made the announcement on Bethenny Frankel's new daytime talk show in an interview to air Wednesday.
"This is my last season for 'Private Practice.' ... It's been an incredible journey, an amazing ride. I'm hugely, hugely grateful," she said.
"It's been a long time being Addison," Walsh said of her character. "I feel incredibly grateful to [writer-producer] Shonda Rhimes and all the fans that have been there from Day 1 to now. I'm really excited about the last season."
Walsh's co-star Tim Daly announced last month he wouldn't be back for Season 6.