O'Donnell, 50, made the surprise announcement on Twitter today, along with a photo of the couple and their new daughter, whom they named Dakota.
"we r thrilled to announce the arrival of r daughter Dakota - #withloveandthanks - AMEN," O'Donnell wrote.
O'Donnell's rep told ABCNews.com that the couple adopted Dakota. She was born on Jan. 5 and weighed in at 5 pounds, 5 ounces. "Moms and baby are happy and healthy," the rep said.
O'Donnell has four kids already - Blake, Vivienne, Parker and Chelsea - from her previous marriage to Kelli Carpenter. Dakota is the first child for Rounds.
The news comes after several months of surprises for the couple. In June 2012, O'Donnell quietly married Rounds. She did not make the marriage public until two months later, when she announced that Rounds suffers from desmoid tumors, a rare and life-threatening disease.
Also in August, O'Donnell surprised fans with the news that she had suffered a heart attack and said it was a "miracle" she hadn't died.