Willoughby, who co-hosts the BBC competition with Reggie Yates, explained that the 'Who You Are' singer pushes contestants and fellow coaches will.i.am, Tom Jones and Danny O'Donoghue to "raise their game".
"Jessie J is incredible. I'm in love with her," she told the Daily Star. "She has such an amazing energy and whenever she's around everybody kind of raises their game. It's going to be an incredible series."
Jessie J recently declared that she is not afraid to be critical about Voice contestants.
"I like how Simon Cowell says exactly what he thinks, whether people like it or not," she explained. "I don't care if people don't like me and I'm prepared for that.
"Even if they are on my team, that doesn't mean I'm going to say it was good if it wasn't. I'm not a robot. I sing out of tune sometimes so I'm not scared to tell someone else if they have. I'm going to be honest. That's what the industry needs more of right now."
The Voice UK will reportedly launch on BBC One on Saturday, March 24.
> The Voice UK Tom Jones, Jessie J, will.i.am, Danny: Meet the coaches - video
> Holly Willoughby: 'The Voice is a feel-good show'
Watch Jessie J talk about The Voice below: