De Niro and Hightower were last spotted attending the Tony Awards together in June.
De Niro was previously married to Diahnne Abbott from 1976 to 1988. Together they have two children, Drena, 47, and Raphael, 42. The actor went on to have two more children, twin sons, Aaron and Julian in 1995 with his girlfriend at the time, Touki Smith.
CNN has contacted De Niro’s representative for comment.