According to onlookers at the city's trendy Lowdown bar, Goodwillie, 21, who has a girlfriend, looked "pretty flustered" when he was caught by staff doing up his zip as he left a toilet cubicle with the girl.
He was enjoying a night out with teammate Danny Swanson hours after their side's 2-0 defeat by Celtic on April 25.
"Goodwillie and Swanson came into the bar and were in pretty good spirits. They were speaking to fans and having a few drinks with them," the Sun quoted the insider as saying.
"Goodwillie was chatting away to one girl for a wee while. After a bit he went into the toilets and she followed five minutes later.
"At last orders the bar staff were trying to clear everyone out of the pub. They checked the ladies' toilets and realised someone was still in one cubicles. One of the girl's friends said it was her pal and she was upset.
"After five minutes the door eventually opened. The girl came out with Goodwillie who was doing up his zip. He looked pretty worked up and was told to leave by staff," the insider added.
Dundee United and the player's agent have both denied the accusations.
"David admits being in the pub but strenuously denies the other allegations," a spokesman for the club said.
His agent, Jim Harkins, added: "There is no truth in this."
But a spokesperson for Lowdown insisted the incident did take place.
"David Goodwillie and a female were asked to leave the pub after being caught in a toilet cubicle together," she stated. (ANI)