The baby is due this summer, Roberts' publicist, Marcy Engelman, said in a story posted on the magazine's Web site.
News of the pregnancy first appeared in the New York Post's Page Six column.
A call to Engelman by The Associated Press wasn't immediately returned.
Roberts, 39, and Moder, 37, have 2-year-old twins, Hazel and Phinnaeus, who were born in November 2004. The couple were married in July 2002 at Roberts' home in Taos, N.M.
She won the best actress Oscar in 2001 for "Erin Brockovich." Her screen hits also include "Steel Magnolias" and "Pretty Woman."
Roberts is the voice of the wise spider in the film adaptation of "Charlotte's Web." She'll next be seen in the drama "Charlie Wilson's War," directed by Mike Nichols and also starring Tom Hanks, scheduled for release next year.
She made her Broadway debut this spring in a revival of Richard Greenberg's "Three Days of Rain."