The 24-year-old actress revealed the news of her engagement to the world heavyweight champion during a show, reported Ace Showbiz.
Panettiere, who had been flashing a major bling on her ring finger since some months, had kept mum about her engagement.
But when she was asked whether she was engaged, she replied, "I am."
The couple, who have been quiet about the status of their on-off relationship, first met in 2009 at a party. After dating for two years, they split for sometime.
The duo reunited in February when they enjoyed a date in Hollywood.