The report claims Sienna and Brad met up at 3am in the festival's Rabbit Hole area where onlookers spied them looking very cosy.
A source claims: "Brad and Sienna couldn't keep their hands off each other, touching and stroking each other at every opportunity. They seemed really intimate."
The insider added that Brad, who up until then was keeping a low profile at the world famous festival, looked delighted to meet up with Sienna, explaining:
"They seemed keen to get away from prying eyes and made a dash for the Rabbit Hole, the festival's underground and exclusive venue, with a group of friends and were inseparable as they waited to go inside.
"The party was cut short and they had to leave after five minutes because the venue was closing. Bradley Cooper left and Brad and Sienna headed back arm in arm to their private Winnebago."
Another onlooker told Daily Star Online: "Brad and Sienna were looking very cosy. I couldn't believe it - they were kissing in the VIP section of the Rabbit Hole."
A representative for Brad declined to comment when approached by MailOnline. Sienna's representatives have also been contacted.
Back in April Sienna lashed out at "silly" reports claiming she was spotted flirting with Brad, who is in the midst of divorcing Angelina Jolie.
The 35-year-old Brit insisted she would not "dignify it with a response", in an interview with the New York Post just days after Page Six claimed the duo enjoyed a dinner date in Los Angeles.
Having previously dated her Alfie co-star Jude Law and enjoyed a "brief" encounter with Layer Cake's Daniel Craig, she conceded that the Brad reports were "predictable", as his production company is behind her new movie The Lost City of Z.
Page Six reported that Sienna and Brad were reportedly seen getting flirty in LA, after which they attended the premiere of The Lost City Z separately but insiders claim at a dinner after the screening the pair got close.
Brad, whose company Plan B is behind the film, was said to be doing some "serious flirting" while surrounded by about 25 people. A source told Page Six: "They were heavily enjoying each other's company and they were talking to each other all night."
It was claimed that soon to be divorced Brad seemed to perk up after the interaction with Sienna as he was "in very high spirits, which he hasn't been for a long time. He seems to be over a hump".
Shortly after the reports emerged, Sienna spoke to the New York Post, where she vehemently denied the reports - insisting she would not entertain the prospect. She said: "I'm not going to even dignify it with a response. It's predictable and silly".
Prior to her denial, another source close to Sienna downplayed the possible flirtation and said the pair were part of a larger group so there were a lot of conversations going on throughout the night.
Back when the movie was filming, Sienna was forced to deny that there was another going on with a then-very married Brad.
She called a suggestion they were flirting "insane" on Sway In The Morning on Sirius XM's Shade 45. The star said: "The brilliant thing about that rumour is that yeah I haven't really read too much about it.
"So this is the truth about that, Brad Pitt is producing the movie that I just wrapped but I didn't see him, he's not been to the set. You know he's the producer of the film and I've met him twice - it's insane."
