The 44-cent stamp was dedicated in ceremonies in Old Saybrook, Conn., and is on sale nationwide.
The red-haired actress becomes the 16th star to be honored on the U.S. Postal Service's Legends of Hollywood stamp series.
During her long career Hepburn won four Academy Awards, the first for her performance in "Morning Glory" in 1933.
Her other Oscar performances were in "Guess Who's Coming to Dinner," 1967; "The Lion in Winter," 1968, and "On Golden Pond," 1981.
Hepburn also received an Emmy for her performance in "Love Among the Ruins," a 1975 made-for-television movie.
"Katharine Hepburn will be remembered for generations, for both her unparalleled acting ability and being a role model for women who chose to live life on their own terms," Postmaster General John Potter said in a statement.