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."
Knapp will soon be seen in the movie "So Undercover."