The 84-year-old publishing mogul took to twitter to announce the engagement - which happened on Christmas Eve (24.12.10) - adding the blonde Playmate "burst into tears" upon seeing The Ring.
He wrote: "After the movie tonight, Crystal & I exchanged gifts. I gave Crystal a ring. A truly memorable Christmas Eve. (sic)"
When questioned on twitter if the ring was part of a proposal of marriage, he said: "Yes, the ring I gave to Crystal is an engagement ring. I didn't mean to make a mystery out of it. A very merry Christmas to all."
Although she did not tweet personally about the engagement, Crystal did thank her online followers for their words of support.
She said: "Thank you to everyone, I really appreciate all of the kind tweets! Much love to you over the holidays. (sic)"
Summing up the festive period, Hugh wrote: "This has been a perfect Christmas weekend. Now it's time to stop tweeting & say goodnight. Sweet dreams."
Hugh has been married twice before - his first wife Mildred, with whom he had children Christie and David, divorced in 1959 after 10 years of marriage and his second marriage to Kimberley Conrad ended in 1998 after a nine-year union.