The 23-year-old Price Tag singer recently spoke to BBC Radio 1's Newsbeat and revealed the news.
She said: 'I met her [Tulisa] at the Glamour Awards and I was contacted by her and I was like: 'You know what? I'd love to do it.' I think it's a really nice way for me to be involved in X Factor.'
Jessie will team up with Tulisa, also 23, at the Judges' Houses stage of the competition and help her decide who to put through to the live finals.
Jessie added: 'I'm definitely someone who is more about the mentoring once people have got through. I'm not really into the whole ridiculing of people who can't sing. That's not really my thing.
'Every single person that's in this industry has worked their butt off to be there and she [Tulisa] has worked her butt off for years and worked her way up the industry and she deserves to be there.
'She's the voice of a group of a young generation of Britain and I'll join her as another female artist and be like: 'Let's decide some groups girl, let's do this.''