Ryan Phillippe was with his ex-girlfriend when she gave birth to their daughter, a source close to the former couple told UsMagazine.com.
The child's name, birthplace and measurements were not reported.
Phillippe, who has two children with his ex-wife Reese Witherspoon, dated Knapp last year. They broke up in September.
Phillippe is best known for his work in the films "Cruel Intentions,""Crash" and "Gosford Park."