Academy award-winning couple Charlize Theron and Sean Penn are reportedly planning to get married.

According to Us Weekly, the couple who have had a friendship spanning 18 years, are also planning to adopt a child together.

"They want to get married very soon," the insider said.

The 38-year-old The Snow White and the Huntsman star, who is already mother to son Jackson, whom she adopted in May 2012, "always intended to have another child."

"Sean is revitalised with Charlize," the source said of Penn.

Reports that the two are dating started when the South African beauty and Penn were clicked together vacationing in a Hawaiian beach on 30 December.

CNN host Piers Morgan confirmed their relationship in a blog post for The Daily Mail's Event magazine, after he attended the 53-year-old actor's Help Haiti Home gala on 11 January.

"I walked over to Sean's table, where he was sitting with new girlfriend Charlize Theron," Morgan wrote. "'I'm so glad you two have got together,' I said, 'Sean's a great guy.'"

"'I know,' she smiled, putting her hand round his shoulder," he added.

Penn was previously married to Robin Wright and Madonna, while Theron was in a long-time relationship with Stuart Townsend. They separated in 2010.

Meanwhile, in the June issue of Vogue, Theron opened up about her relationship with Penn.

"What I guess I am trying to say is: a friend kind of stepped into that role, and I really didn't see it coming. Sean and I have known each other for 18 years. We were just really good friends. And it's been slow going because you are aware that you could screw that up and lose it. We had to think about it," she told the magazine.

"But at the end of the day, it was somewhat effortless. It just kind of naturally happened, and before I knew it, I was in something that was making my life better - the people who really love me can see the effect it has had on me," she added.