Kylie, who was a coach on The Voice UK with Sir Tom on the 2014 series, joins her friend to give advice to Team Tom ahead of their crucial Knockouts performances next month.
The two superstars shared their knowledge during piano rehearsals when each act performed for them.
During her three decades in the music industry, Kylie has notched up seven No.1 singles and 80 million album sales. Her latest single Dancing is out now while her 14th studio album Golden is released on April 6.
Sir Tom said today: “I have known Kylie for a long time so it is nice to have someone you know personally giving you help.
"It is reassuring having a friend as well as such an experienced professional.
“It was tremendous working with her when we were both coaches on The Voice UK in the past. She is an accomplished artist and a really lovely person.”
Kylie added: “It’s so great to be back amongst The Voice family as a guest mentor for Tom Jones. I love working with Sir Tom and have been blown away by the talent in his team this series.
“What a pleasure to work with such a great group of contestants and it’s fair to say I’ll be keenly following the rest of the series. Good luck Team Tom!"
Jennifer Hudson’s mentor will be revealed soon.
For now, The Voice UK 2018 continues this Saturday night with the final round of auditions.
The Voice UK airs at 8PM on Saturday.
Following the auditions are the Battles followed by the Knockouts, with the guest mentors.
Each coach will take six acts into the Knockouts but only two per team will make it through to the Semi-Final.