The model announced the pregnancy on Instagram a few days after multiple reports speculated that she was expecting.
In sharing an idyllic photo taken by her beau, a bikini-clad Rosie is seen on a beach with a fairly pronounced baby bump.
"Very happy to share that Jason and I are expecting!!" she captioned the image. "Lots of love Rosie x."
On Feb. 7, an insider told Us Weekly that Rosie also looked "noticeably pregnant" at a recent photo shoot.
Speculation of a possible pregnancy actually started on Jan. 2 after the "Transformers" actress shared an Instagram image of her petting a horse. "Here's to fresh starts and exciting prospects for the new year. Wishing everyone a happy and healthy 2017..❤," she captioned the image.
She was also photographed in late January at LAX in head-to-toe black outfit and an oversized trench coat, concealed her tummy, although it did peak through at times. Other recent photos showed her covering up her stomach with coats or handbags.
It will be the first child the "Mad Max: Fury Road" actress and her action star fiance.
"Having a family is something I think about for sure, and whether I'd live here in America or England. But it's not always as simple as that," she said in 2015. "I have no idea what will happen, but it'll be interesting to find out, right? And nothing will be perfect, I'm sure."