The actress gave birth to her first child with fiance Benjamin Millepied, the choreographer of "Black Swan," People magazine reported.
The magazine gave no details. A publicist for Portman did not return a message seeking comment Tuesday.
The 30-year-old actress and Millepied, a well-regarded ballet dancer and choreographer, met during the making of "Black Swan," Darren Aronofsky's psychological thriller that stars Portman as a ballet dancer.
Portman won the best actress Academy Award in February for her performance in the movie.