The actress announced the news on her Instagram Thursday afternoon with the caption, "Guess what guys! @matthewkoma and I made a little princess of our own and we couldn't be more excited!!!!!"
Hilary Duff, 30, has a 6-year-old son with her ex-husband, retired NHL center, Mike Comrie. This is the first child for Duff and her boyfriend, singer,
Matthew Koma. The couple has been dating on-and-off for several years.
In December, Duff shared on "The Talk" that she and Koma had gotten back together.
"It's going so great," she said. "[It's] the third time that we've dated. Timing is such a big deal ... third time's a charm!"
Koma also took to Instagram to post his own sweet message.
"We made a baby girl!" he captioned a photo of himself kissing Duff. "She will be as beautiful and sweet as her mother... @hilaryduff another incredible chapter begins."
Duff's sister, Haylie Duff, also announced on Thursday that she had welcomed her second child, Lulu Gray, with her fiancé Matt Rosenberg.