Youngsters grab trams' windscreen wipers and stand on their rubber bumpers while the vehicle is stationary.
Trams then travel at speeds of up to 30mph, often along busy main roads, with the thrill-seeking youths clinging on the back.
Today Supertram chiefs warned it is "only a matter of time" before a child is killed while balancing on the back of moving trams in the daredevil stunt.
The warning follows shocked eyewitness reports of the latest tram-surfing incident, on Infirmary Road in Upperthorpe. In the last year there have been up to 10 reported incidents and two arrests.
One horrified onlooker told The Star: "I couldn't believe it. We were travelling at quite a high speed. If one of them had fallen off they wouldn't stand a chance."
The woman said she was sitting at the back of the tram when three "rowdy youths" got on.
"As we were travelling along Infirmary Road one of them shouted to the conductor that there were tram-surfers on the back," she said.
"The conductor made an emergency call for the driver to stop. When he did, two lads ran off - they must have been hanging onto the back of the tram.
"The youths on the tram were just laughing. The conductor said to one of them he wouldn't be laughing if one of the lads died - the youth just said he would."
Julia Shaw, commercial manager at Stagecoach Supertram, today said she feared it was "only a matter of time" before Sheffield witnessed a tram-surfing tragedy.
"It takes an adult imagination to think what could happen," she said. "To youngsters it's a free ride that's a bit more exciting. But they don't realise the dangers behind it.
"They could come off at a swing into the path of a tram or car coming in the opposite direction. I'm quite sure they wouldn't be laughing if one of them got killed."
The problem is difficult for police because tram drivers can't see whether tram-surfers are hanging onto the backs of their vehicle.
"Sometimes a conductor sees them, or quite often it's passengers that report them, or passing trams," said Julia.
The company liaises with tram operators in other cities to discuss the problem, and sends speakers into schools to highlight the dangers of tram-surfing.
Supertrams have CCTV cameras installed inside as well as cameras on the front and back. "The cameras are not obvious to the people doing the surfing," said Julia.
"They don't realise they are looking straight at the camera."
Supertram operations manager Eddie Birkitt said there had been between five to 10 reports of tram surfing in the last year.
"There have been various incidents around the city including Middlewood, Shalesmoor, Fitzalan Square and Park Grange Road, but they are just the ones we know about.
"We had two people arrested for tram-surfing last year.
"It's a very stupid practice," he added. "If the tram has to brake suddenly it would be very difficult for the youths to hold on and they could be catapulted off."
In December 2001, a 16-year-old from London was killed instantly when he fell from a moving tram while surfing. He was run over by two trams before his body was found. The same year a 17-year-old boy from Bolton suffered serious head injuries in a similar incident.
In 2000 a 19-year-old man from Croydon was treated for head and arm injuries after tram surfing, and was arrested by British Transport Police.South Yorkshire Police traffic management officer Ian Shelton urged parents to be aware of where their children are and what they are up to.
"Tram-surfing is an extremely dangerous stunt, with the likelihood of people being critically if not fatally injured," he said. "Any incidents where people are seen to be hitching a ride on a tram will be investigated."

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