People.com quoted an unnamed source as saying, without elaboration, that Witherspoon and Gyllenhaal recently ended their romantic relationship after about two years.
However, representatives for the celebrities told the magazine's Web site the insider has the story wrong.
"It is not true," the stars' publicists Nanci Ryder and Carrie Byalick said in a joint statement. "They are still together."
Witherspoon, a divorced mother of two, won the 2006 Oscar for her work in "Walk the Line."
Gyllenhaal is best known for his performance in the 2005 film "Brokeback Mountain." His new film "Brothers" is to open in U.S. theaters Friday.