The socialite and the 'Confident' singer are both currently in the South of France for the annual film festival and got close when she reportedly sat on the younger star's lap at Gotha nightclub.
An onlooker told the Daily Mirror newspaper: "Paris had a private table above the DJ setup at the nightclub and was clearly loving being out and being the centre of attention. She was trying to dance sexily and looked really up for a party."
Paris - who is dating model River Viiperi - wasn't too happy about people taking photos, with the source adding: "She hit the roof when people started filming her and she was yelling, 'I can't be filmed by a rival club - I'm being paid a lot of money to appear in another club!'
"Her people stepped in to calm her down and persuade her to return to her table, where she then started dancing, sucking on a lollipop and flirting with Justin again."
The 33-year-old hotel heiress also reportedly left with the 20-year-old singer and his entourage to carry on their party back at his mansion, but wasn't too impressed with his musical choices and left soon after.
A source added: "Paris ended up leaving the party after half an hour when Justin was just playing his own songs. She said the party was really lame."