Hilary Duff Having a Baby Girl with Boyfriend Matthew Koma

Hilary Duff is adding to her family -- she's pregnant with her second child ... and this time it's a girl.

Hilary just announced her pregnancy, saying she and her bf -- musician and songwriter Matthew Koma -- "made a little princess of our own and we couldn’t be more excited!!!!!!" It will be the couple's first kid together.

Hilary has a 6 year old son named Luca Cruz Comrie with her ex-husband, retired pro hockey player Mike Comrie.

As we reported ... Hilary and Matthew got back together in late 2017 after splitting in March, and it was actually their third try at dating. Duff said, "Timing is such a big deal ... third time's a charm!"

Looks like she was right. Congrats!!!