Zolciak, who broke the news to Life & Style, is four months along and due in late summer. It's the couple's second go-round at parenthood, after welcoming the arrival of son Kroy Jagger last June.
"Kroy and I are so excited," Zolciak, 33, says. "To decide we wanted to get pregnant again and have it happen so quickly was such a blessing."
No word whether the new bundle of joy will be a boy or a girl.
Zolciak also has two daughters, 15-year-old Brielle, and 9-year-old Ariana, from a previous marriage. She met Biermann, a 26-year-old defensive end for the Atlanta Falcons, at a charity event in May 2010. The twosome tied the knot in November.