The 18-year-old actress - who was recently pictured holding hands and kissing the `Baby` hitmaker - did not meet the Canadian heartthrob until his management got in touch to ask to meet.
Discussing how long she knew Justin, Selena said: "Uh, almost, like, two years now. His manager called my mom, who`s my manager."
Although neither has yet officially confirmed they are dating each other, Selena "blushed" when asked further about their relationship and said it was "nice" to have him around.
She told US TV talk show host David Letterman: "I`m blushing! He`s been in my life for so long and it`s just nice to have someone that understands what you`re going through."
Selena recently confessed she was "hurt" by Justin`s fans who criticised her via social networking site twitter, but has her fans and family who keep her strong.
She said: "It`s hard. It hurts, it really does. I don`t feel like I`m doing anything wrong. I`ve been best friends with him for a very long time. It does hurt my feelings a lot but I try not to focus on it. I have a strong family and great fans."