The model mama-of-one took to Instagram to announce the exciting news.
"Cooking in the oven," she penned alongside a snap of her bub's ultrasound.
"Tears of joy as bubby no.2 is on the way.!!! 12 weeks along!! (pic from our 8 week scan). Excited to share this new adventure with you guys!"
The 28-year-old and her hubby Michael Miziner are already parents to their two-year-old daughter Violet, who they welcomed in September 2013.
Speaking candidly to The Sunday Telegraph on the joys of motherhood, the Aussie beauty admitted a healthy balance is essential for her sanity.
“Every weekend (Violet) goes to Mish’s mum’s house and we get our weekend to ourselves,” she said.
“I think that’s incredibly healthy for the relationship.”
Rachael became a household name as the 2009 Miss Universe Australia entry.
The following year she cemented her place in the spotlight as a contestant on Dancing With The Stars, where she famously rumba'ed her way to love with now hubby, Michael.
The pair tied the knot the same year they welcomed their firstborn.
A huge congratulations to the family!