"Today we have big, exciting, wonderful, magical, amazing news!" she wrote. "Matty and I are expecting a baby girl!"
Haylie Duff and Matt Rosenberg are expecting their first child - a baby girl
Describing her pregnancy as "the moment that has changed my life forever", Haylie added, "Entering this new chapter of our lives and our relationship has been so special. We are so happy and so in love with her already!!!"
The star made the announcement alongside a picture of some tiny black, white and pink Jordan trainers.
Haylie Duff announced her pregnancy news alongside this photo
Haylie's younger sister Hilary Duff was quick to celebrate the happy news. "GUYS I'm going 2 be an auntie," tweeted the 27-year-old, who has a two-year-old son Luca with estranged husband Mike Comrie.
"@haylieduff & @matt_sts are going to be the best parents to a sweet baby, & luc will have a partner in crime."
Hilary Duff was among the first to congratulate the parents-to-be
Haylie and Matt announced their engagement in April, and on 22 November celebrated their two year anniversary together in Hawaii. To mark the occasion, Haylie shared a romantic photo of the soon-to-be parents smiling on a beach, which she captioned, "Happy Anniversary to the greatest man I know. I'm lucky to be yours. @mr_sts."