The 84-year-old Playboy founder has been married twice before, officially divorcing his second wife in March of this year.
In the early hours of Christmas Day morning, he wrote on his Twitter page: "After the movie tonight, Crystal & I exchanged gifts. I gave Crystal a ring. A truly memorable Christmas Eve."
Later in the evening, he posted again, writing: "When I gave Crystal the ring, she burst into tears. This is the happiest Christmas weekend in memory."
24-year-old Harris, born of British parents living in Arizona, was noticed by Playboy representatives while she was studying psychology at San Diego State University.
She met Hefner on Halloween in 2008 and started dating him a few months later, while he was also dating identical twin glamour models Kristina and Karissa Shannon.
The magazine tycoon since ended his relationship with the twins in January this year, and has remained monogamous to Crystal since.