"Yes, the ring I gave Crystal is an engagement ring. I didn't mean to make a mystery out of it. A very merry Christmas to all," the 84-year-old wrote on his Twitter account.
"When I gave Crystal the ring, she burst into tears. This is the happiest Christmas weekend in memory."
Hefner said he celebrated Christmas Eve with his family and friends at the Playboy Mansion where they watched the film "The Tourist."
"After the movie tonight, Crystal & I exchanged gifts. I gave Crystal a ring. A truly memorable Christmas Eve," Hefner wrote following his Christmas Eve marriage proposal to his 24-year-old girlfriend.
Hefner had bad luck with his first two marriages to Mildred Williams, who he divorced in 1959, and to former Playmate Kimberly Conrad -- who he has been separated from for almost 20 years but only formally divorced in 2009.
The two failed relationships resulted in Hefner's hesitation to wed again, as he wanted to take things slow with Harris -- who was December 2009's Playboy Playmate -- in hopes of having a different outcome.
"I haven't had a lot of luck with marriage," he told E! News in January. "I don't have a good track record and I don't want to screw this one up."
But Hefner's fear of commitment didn't just start with Harris. He also showed signs of delay with his previous girlfriend and former The Girls Next Door star Holly Madison. The couple split in the Fall of 2008 due to similar factors in which Hefner didn't want to get married or have children.
Madison often hinted to Hefner her plans to become husband and wife and make a family, but Hefner made it clear he didn't see it in his near future.
"I love Holly very much and I think we're going to be together the rest of my life. But marriage isn't part of my puzzle," Hefner told OK! Weekly in 2008.
"It's not a personal thing. I just haven't had much luck with marriages."
The Girls Next Door's fifth season ended with Madison and two of Hefner's other former girlfriends Bridget Marquardt and Kendra Wilson all revealing plans to move out of the Playboy Mansion shortly after Madison and Hefner broke it off and Wilkinson got engaged to NFL star Hank Baskett.
Before the show's sixth season took off, 19-year-old twins Karissa and Kristina Shannon and Harris -- who was 23-years-old at the time -- moved into the Mansion with Hefner, while Madison, Marquardt and Wilson all departed.
However, during The Girls Next Door's sixth season, the Shannon twins moved out of the Mansion, a move that redefined them from Hefner's girlfriends to just plain friends -- leaving Harris and Hefner a chance to become more serious and grow in their relationship together.