Tickets for The Who's Swansea show are being advertised on internet sites at more than twice their face value - even though the gig is not sold out.
Roger Daltrey and Pete Townshend are The Who's surviving members
The band will play the first major concert at the city's Liberty Stadium next Friday.
Stadium mangers say tickets are still available, despite touts "cashing in", and urge people not to be "ripped-off".
They said tickets priced at £40 to £55 were being offered for sale at close to £120 on some auction web sites.
Stadium general manager Andrew Davies said: "It seems to be the case with every major event these days that there are people looking to cash in and make a quick buck out of it.
"But I would like to reassure anyone who wants to attend Friday's concert that has yet to buy a ticket, that they are still available to buy from official outlets.
"There is no need for anyone to get ripped off paying over the odds for their tickets."
The Who were one of the 1960s' biggest bands and are still a major draw today, although they are now reduced to two original members - guitarist Pete Townshend and singer Roger Daltrey.
They will be joined on the bill by '90s indie band The Charlatans, and Killing For Company, the new group put together by ex-Stereophonics drummer Stuart Cable.
Mr Davies said "more than two-thirds" of the stadium had been sold out, so "there is sure to be a great crowd and a fantastic atmosphere".