The 'Only Girl (In The World)' singer took to her Twitter account earlier this week to say that she saw Bieber's abs while they dined together in New York.
"Justin Bieber just flashed me his abs in the middle of a restaurant! Wow! He actually had a lil 6 pack! Sexy,lol! #Beliebersplzdontkillme, (sic)" she wrote.
Speaking on Ryan Seacrest's radio show yesterday, Bieber said: "We all went out for dinner, the label and everybody, and Rihanna was there.
"We sat together and I said a couple of lines to her!... I don't remember [what I said], I was just being funny. I don't remember how it came up but she was like, 'Do you have abs?' and I said, 'Uhh, yeah...'
"She said, 'Show 'em!' and everybody was like, 'Yeah!' so I did and she tweeted!"
The teenager later responded to Rihanna's tweet with a message of his own, saying: "Great night out to dinner with the beautiful @rihanna... I'm not complaining... RiRi....check for the new album (sic)."
Bieber, who will perform at this weekend's American Music Awards, was on the show to promote his upcoming record My Worlds Acoustic which will be available at Walmart stores from November 26.
Seacrest's radio show airs weekday mornings on KIIS-FM.