The "Umbrella" singer - who has previously denied dating the actor, saying they are just good friends - knew they were made for each other when they met partying in New York last October.
She said: "It's good to be friends first. I don't find it too hard to talk to guys - I have a million guy friends."
"It helps if you have things in common, but if someone is right for you, you'll know it. When I hang out with Josh, it feels right. He's lovely."
The 19-year-old star explained she is still cautious about getting too close to Josh, after getting her heartbroken when she was younger.
She said: "I had my heart broken when I was 18. It's never easy to get over that, but you know, you've just got to get on with life."
Rihanna also revealed she likes to test her suitors by playing hard to get.
She added to Top of the Pops magazine: "Well, I always keep my guard up with guys and I guess that can get in the way sometimes. I can make them go thought hell!"
