Duff, 33, shared the exciting news on Saturday by posting a Boomerang of her growing baby bump and the musician rubbing her belly as he hugs her from behind.
"We are growing!!! Mostly me ...," the former Lizzie McGuire star captioned her post. Duff and Koma are already parents to almost-2-year-old daughter Banks Violet. The actress is also mom to 8-year-old son Luca, whom she shares with her ex-husband, hockey player Mike Comrie.
Duff's friends and followers congratulated her on expanding her and Koma's family.
Amanda Kloots wrote, "Omg congratulations this is amazing news!" Christian Siriano added, "Congratulations!!" and Jamie Lynn Sigler commented, "Oh my gosh congrats!!!!!!"
"Good luck winning a fight with your wife when her name’s tattooed on your butt cheek. #squats #gainz #laseraway," Koma captioned a pic showing "Hilary" written in cursive on his left cheek.
"Finest tiniest booty around," Duff wrote on his post. "#YouStuckNowBoy."
Duff joins a long list of celebrities who are expecting a baby. Ashlee Simpson, Bindi Irwin, Ashley Tisdale, Emma Roberts and many more are adding a family member into their lives. See who else, below.