It was the first of a number of videos Rebel uploaded in her costume, including one doing a warrior-themed dance with her sister Annachi, TV presenter Marissa Montgomery and stuntwoman Meredith Richardson.
"TOGETHER WE ARE: THE NUN-CHICKS," Rebel wrote in the caption.
She was later joined in the festivities by her boyfriend Jacob Busch, who popped up on her stories topless in a pre-party selfie.
Another photo of the pair suggests it wasn't the only Halloween party they headed to this weekend — though the second seemed a slightly more formal affair, with Rebel looking demure in a black T-shirt dress with sheer detailing and creepy skull face paint.
Rebel and Jacob were introduced last year, but only started dating when the comedian returned to the US after quarantining in Australia during the Covid-19 pandemic.
"They are officially boyfriend and girlfriend," a source told People magazine last month.
"He's a gentleman, has very old school manners and treats her well.
"He is also very health conscious and they've been motivating each other with their health journeys."
They made their red carpet debut as a couple at Prince Albert's Planetary Health Gala in Monaco in September.
The handsome fella, 29, is a member of Anheuser-Busch brewing dynasty and is rumoured to be worth more than $150 million.
Rebel is nearing the end of her "Year of Health", with the actress looking slimmer than ever having revealed earlier this month she was just 3kg away from her 'goal weight'.
The Pitch Perfect star, 40, shared a celebratory picture following an early morning hike and said she felt "proud".
"Happy Sunday everyone! Gearing up for a great week ahead," she wrote.
"This week was super busy but I got up super early 3 times (6am) and went on a hike … even did a couple of 100m sprints to get the heart rate even higher (although my 'sprint' is probably someone else's 'slow jog').
"But I felt proud of myself and now only 3kgs away from my goal weight! Rebs x".
The star has been dedicated to her exercise regime and healthy nutrition as she dropped dress sizes.
She promised at the start of the year to start a new fitness journey and has been true to her word.
Rebel made a New Year's resolution to ditch her unhealthy eating habits and kickstart a new fitness journey.
At the time, she wrote on Instagram: "Okay so for me 2020 is going to be called 'The Year of Health'."
To the delight of fans, she's been documenting her progress on Instagram.
