The 'Bang Bang' hitmaker said she felt the need to support Bieber, 20, because of the immense pressure he is under and the attention his fame brings to him, reported Contactmusic.
"I just felt like it was my duty. I hand-wrote it, old school. I was just like, 'I'm here, you're amazing.' He's young and he's got so much pressure on him. It's easy for people to comment when he's having a photo taken of him, but you don't see what we see.
Jessie, 26, eventually made the decision not to post the letter as she felt it might be inappropriate to send it to someone who she doesn't know.
"I just didn't think it was my place. I know people who know him, so I could have got it (the letter) to him. But instead I just made sure his people knew I was available to speak to m as I have been to many artists," the singer said.