The "Umbrella" singer - who was spotted "kissing and cuddling" Josh in a New York nightclub last month - has confirmed the pair are dating.
Speaking at the World Music Awards in Monte Carlo, Rihanna said: "I would be lying if I told you we were not more than just friends."
"I have so fallen for him, he is lovely. He is so hot and he is really sweet to me. When we hang out it feels right - even though it's still pretty new."
The pair first sparked romance rumors four weeks ago when they seen intimately enjoying each other's company as they shared a table and drank Dom Perignon champagne at New York nightspot Pink Elephant.
Josh reportedly decided to make his move on the 19-year-old R'n'B beauty after watching her perform in her racy stage outfits at the Nokia Theater in Times Square the night before.
The 29-year-old actor refused to deny rumors he was romancing Rihanna and dodged questions about the couple's steamy nightclub embrace during a subsequent TV interview.
When asked if he had been kissing Rihanna, Josh stuttered: "Whoa, whoa, whoa! No, no, no. Come on you guys! This is, uh, we weren't supposed to talk about these things were we?"
Josh has previously been linked to "Black Dahlia" co-star Scarlett Johansson and supermodel Helena Christensen.
Rihanna was recently said to be dating "Transformers" star Shia LaBeouf.
