The Unforgettable actor announced that daughter Amelie Belle was born in late September, though it is unclear who the baby's mother is.
"My best moment of 2011 would definitely be the birth of my daughter six weeks ago, on September 25," Walsh told People.
"[She's] six weeks old. Not a whole lot of sleep happening at my apartment."
Walsh is also father to 15-year-old Thomas and 14-year-old Joanna from his marriage to ex-wife Melora Walters, as well as 8-year-old Stella with first wife Joanna Going.
Walsh filed for divorce from Going in December 2010 after six years of marriage.