The model, 29, took to Instagram to share the happy news with her 6.8 million followers, posting a picture of herself on a picturesque beach showing off her bump in a bikini.
'Very happy to share that Jason and I are expecting!! Lots of love Rosie x ❤👼🏼 Photo by @jasonstatham', she wrote alongside.
Fans were quick to shower the pair with congratulations, with one writing: 'This child will be the most beautiful' and another adding: 'Wow incredibly exciting!! I love the two of you. My favourite couple.'
'Aw congratulations!! Couldn't happen to a nicer couple 😄' a third agreed.
Rumours of Rosie's pregnancy had begun to circulate after she was pictured wearing increasingly baggy clothes, but this is the first time that the notoriously private couple have spoken publicly about their growing family.
The child will be the first for both Rosie and actor Jason, 49.
Rosie had made no secret of her desire to become a mum, saying during a previous interview with ELLE magazine: 'I always see myself working, but definitely being a mum will be a big part of me.'
'I think there is naturally a point where you think, "I don't care about myself as number one any more. I want to care about other things."'
Rosie and Jason have been dating since 2010 and announced their engagement in 2016, with Rosie sporting a diamond on her ring finger on the red carpet at the Golden Globes - although the couple have not yet revealed when they intend to tie the knot.
In 2015, Rosie said to SELF magazine of her then-boyfriend: 'We're best mates.'
'He makes me laugh every day. I almost feel like I never had a proper boyfriend before!'