The Pitch Perfect star checked herself in to The Ranch Malibu fitness retreat in California last week (ends22Apr16), and by Monday (25Apr16), she was already celebrating her rapid slim down.

"OMG (oh my God) just finished 4 fantastic days at #TheRanch4.0 ...so challenging but very rewarding!" the 36-year-old shared on Instagram, as she posted a photo of herself mid-hike.

She continued, "Not to mention I lost 8 pounds from marathon hiking over the 4 days! Thanks to the amazing staff and masseuses (free daily massages are part of the program)!! Feeling great!!"

Rebel isn't the only celebrity fan of the relaxation retreat - fellow actresses Lea Michele, Minka Kelly, and Julianne Hough have all previously enjoyed escaping to the spa resort, where guests are required to leave their cell phones behind and take part in scheduled work outs and hikes, all while dining on healthy plant-based meals, according to People.com.