In a message to his Twitter followers on Christmas Day, Hefner said he had given his 24-year-old girlfriend, model Crystal Harris, a ring.
“Yes, the ring I gave Crystal is an engagement ring,” he clarified in a later tweet.
Hefner set up Playboy magazine in 1953, and Harris featured in it as Playmate of the Month in December 2009.
The soft-porn magnate and libertarian has been married twice before – to Mildred Williams in 1949, and Kimberley Conrad in 1989.
Over the decades he has admitted affairs with dozens of Playmates.
But last April Hefner and Harris reportedly “became exclusive”.
“When I gave Crystal the ring, she burst into tears,” Hefner told Twitter followers.
“This is the happiest Christmas weekend in memory.”
For her part, Harris told Twitter followers simply: “The most memorable Christmas ever.”