The 40-year-old Jackass daredevil, who lost his close friend Ryan Dunn in a fatal car crash earlier this summer, welcomed his second child with wife Naomi Nelson on October 6.
"Her name is Arlo and mommy and baby are doing great," Knoxville tweeted Friday.
The newborn joins older siblings Madison (from Knoxville's former marriage to Melanie Lynn Clapp) and Rocko, nearly 2.
Knoxville and Nelson wed on September 24, 2010. He popped the question with a 2.5 carat Edwardian engagement ring from L.A.'s Platt Boutique Jewelry.