Chris - who was arrested after allegedly attacking Rihanna last month - is believed to have proposed to the "Umbrella" singer during their emotional reunion at Diddy's Miami mansion last weekend.
A source said: "They had a tearful face-to-face meeting at Diddy's, during which an emotional Chris apologized, begged for forgiveness and proposed to her on the spot."
"She was totally up front and confessed to him, 'I can't live without you.'"
After Rihanna accepted the 19-year-old star's proposal, the couple asked a minister to marry them at Diddy's $14.5 million mansion on the exclusive Star Island.
Earlier this week, it was revealed Rihanna's family were concerned about her reconciliation with Chris.
However, the 21-year-old beauty has seemingly ignored her loved ones' advice and is already planning a family with the "Run It" singer.
The source added to America's Star magazine: "All she's ever wanted was to be with him forever."
"Rihanna is looking for the husband-and-two-kids deal before she turns 25. She believes in fairy tales, and she wants to live hers with Chris."
The couple are still hiding out together in Miami.
Chris' arraignment is set to take place in Los Angeles today (March 5, 2009).
He is alleged to have beaten and choked Rihanna until she was unconscious after the pair got into an argument in his rented silver Lamborghini car on February 8.
