Art student Kevin Whitney made the movie, titled Psychedelia, in 1969, and the 22-minute silent film features a series of seemingly random faces.
Barrett features in a 60-second long clip, and the film is now being put up for sale after gathering dust in storage for 44 years.
Whitney tells Uncut magazine the footage was shot at a house in London where several rock stars and actors were frequent visitors, saying, "Syd was part of the scenario. Anybody that would agree, I got to appear in the film... I just had a camera and some psychedelic lighting. It was very amateur and everyone was stoned. I'd tell people to do what they wanted...
"I filmed it because they were fabulous people... Most people were too in awe of Syd to ask to film him. I think that comes across in the film. People who knew him said he looks so happy. He was a very troubled mind and this wasn't a great time."
The film will go on sale at the Room & Book event in London in June (14).
Pink Floyd expert Will Shutes, who is selling the footage, adds, "Syd was so well known but there's very little footage and Pink Floyd usually buy it. This was made in 1969, he was already in a bad way and so it's extremely difficult to find anything like this."
Barrett died in 2006, aged 60, after a battle with pancreatic cancer.