The 33-year-old actress is expecting her first child with husband Adam Shulman and decided to treat her fans to a shot of her blossoming baby bump on the social networking site.
The snap shows Hathaway on the beach wearing a red two-piece, tenderly cradling her stomach with a huge smile on her face.
She wrote alongside the picture: "Happy 2016 to my beautiful Instafriends! So, posting a bikini pic is a little out of character for me, but just now while I was at the beach I noticed I was being photographed.