The Anger Management star, 48, popped the question on Saturday while the couple were enjoying a romantic Valentine's Day getaway in Hawaii.
Charlie, who has been married three times before, took Brett, 24, by surprise when he proposed shortly after the lovebirds had watched the sun rise.
"We are engaged," Brett told E! News. "He even was a gentleman and got down on one knee. I was not expecting it at all.
"The ring is classic and beautiful, but I don't have any photos to share just yet. This is the happiest moment of both our lives. He's my best friend and my soul mate."
Adult film star Brett gushed over her fiancé the day before he proposed, tweeting, "Happy Valentine's day to the love of my life & my best friend! Everyday is Vday when I'm with you @charliesheen."
The couple first went public with their romance last November when Charlie and Brett were pictured canoodling on the balcony of their hotel in Cabo San Lucas, Mexico.
Earlier this year, Charlie sent the rumour mill into overdrive when he pretended that he and Brett — nicknamed Scottie — had tied the knot during a holiday in Iceland.
The hell-raising actor posted a picture of the couple together outside the former French consulate building in Reykjavík, where a 1986 Cold War summit was held between Ronald Reagan and Mikhail Gorbachev.
"'This is the house where M Gorbachev R Reagan did some epic. It's also where S and I GOT MARRIED!" he captioned the shot. Charlie added the hashtag, "RumorMilUhPede".
His beloved only added to the suspicion that it was all a big joke with her comments. "Hahahahahahahahahahaha epic," she responded when one of her Twitter followers talked about the nuptials.
This will be the fourth trip down the aisle for Charlie. The father-of-five had his first brief marriage to Donna Peele annulled in 1996. Charlie then married Denise Richards in 2002 and Brooke Mueller in 2008, both of which ended in divorce.