The 'Piece of Me' hitmaker remembers meeting the 'Baby' singer when he was just 16-years-old and has praised him for being "unassuming".
Speaking to Scott Mills on his BBC Radio 1 show, she said: "He walked into my hotel room and he was like a kid. Literally he looked like he was 13 years old but he was 16. And I was like, 'Who is this kid in my hotel?'
"And my sister was like, 'This is Justin Bieber and his mother.' And I was like, 'Oh my God, I cannot believe this.' He's very unassuming. He's a very, very nice, kind boy."
Meanwhile, the 34-year-old singer previously admitted she feels sorry for the 22-year-old star because of his continued exposure in the spotlight.
She said: "Whoever is in the spotlight, people are really quick to judge. There are a lot of kids coming up who've experienced that. You know, Justin Bieber, he's huge, and he experiences that. It's just the way the world works, unfortunately ...
"I've been doing this for 20 years. I kind of don't pay attention to it. I have my relationship with God and myself and that's what matters to me. I really don't care what most people think."
Meanwhile, it was previously reported that Britney has enlisted the help of a "swagger coach" to increase her confidence.
A source said: "Britney's been gradually getting better for years and being in Vegas is a big help for her state of mind. You can see the difference in how she was in those first shows in 2013 to now.
"But it's her new life coach - who she calls her 'swagger coach' - that's helping her take things to the next level. She won't tell anyone what they do or who he is, but it's working. She's more confident than she's been in forever.
"And after years of going from one boyfriend to the next, she's enjoying being single - she feels empowered and like she can do what she wants."