The "Jackass" star said he and his wife are "having another little bambino in the fall," UsMagazine.com reported.
"My 15-year-old daughter Madison is very excited about it," Knoxville wrote on his official Facebook page. "And her 16-month-old little brother Rocko has, well, no clue. Wahoo!"
The couple wed last year. Madison is Knoxville's daughter from a previous marriage.
In addition to starring in the "Jackass" TV show and movies, Knoxville has also acted in "Men in Black II" and "The Dukes of Hazzard."