The Geordie Shore star posed in nothing but her mismatched undies for a mirror selfie on Friday (22 January).
As she took the snap, Marnie's slender arms, trim thighs and flat stomach were clear to see.
The star, 24, has clearly been taking tips from fellow Geordie Shore girls Holly Hagan and Charlotte Crosby.
Both self-confessed fitness and health fanatics, Marnie's co-stars have been busy building their fitness empires.
Marnie's been talking about her own "progress" and from the look of this latest snap, we reckon it won't be long before she follows suit!
In fact, Marnie's even been using Charlotte as inspiration.
Marnie took a picture of Charlotte's latest fitness DVD, 3 Minute Bum Blitz, and thanked Charlotte for sending her a copy.
"Loving my #3minutebumblitz thanks @charlottegshore it's amazing," said Marnie.
She's even been showing off her bum in yet more sexy selfies!
All these snaps are bound to be making Marnie's former fiance Ricky Rayment, off of TOWIE, super jealous.
The pair ended their engagement last year and Ricky has since been kissing Ex On The Beach star Nancy May.
Meanwhile Marnie hasn't held back when it comes to her thoughts on the new romance.