The Felicity actress welcomed her second child on December 27.
A rep for the 35-year-old star confirmed the news to Us Weekly.
Russell gave birth to daughter Willa Lou Deary, the second child for her and husband Shane Deary.
“I just feel like child birth is one of the most powerful things that happen to a woman,” she told the New York Observer.
The couple, who married in 2007, are already parents to son River Russell Deary, 4.
The actress can next be seen in the romantic comedy Austenland that is set to come out this year.