The former Baywatch star began dating the Welsh beauty earlier this year (11) after they met on the set of U.K. TV show Britain's Got Talent, where Hasselhoff sits as a judge.
The actor recently whisked Roberts away for a romantic break in Cape Town, South Africa, where he decided to pop the question to his new love - only to be turned down.
Hasselhoff then posted a photo of himself on bended knee in front of the blonde on his Twitter.com page, with the caption, ""I tried to pop the question but she was having none of it... Where should I pop the question next?""
The 59 year old later offered Roberts his hand in marriage in a backstage dressing room, but he was rejected once more.
He writes, ""I popped the question in the dressing room. She's still having none of it!""