Devon and Cornwall Police have discovered 14 such establishments in Newquay alone since July.
Officers say sex workers typically rent the properties out for a week, advertising their services online, before packing up and moving on.
Det Insp Dave Meredith said he believed sex workers were being trafficked into the seaside resort by organised gangs.
Speaking on BBC R4's PM, he said it was virtually impossible for owners to know about the activities taking place in their holiday homes.
The problem is not believed to be confined to Newquay, Mr Meredith said, adding it was proving difficult to disrupt.
More on the 'pop-up brothels' and other stories
"It's very probable that there's organised crime behind these sex workers and that organised crime is certainly skimming off some of the money they make," he said.
One guesthouse owner, who gave her name as Ellen, said the first she knew about a temporary brothel on her street was when a man knocked on her front door last July.
"He asked me how much I charged, assuming I was part of it. I just told him to go away - I was shocked," she said.
She added that she had a duty to protect her guests and, with young families living nearby, it simply was "not what you want in the area".
The force is urging residents in Newquay and people living alongside holiday lets to be vigilant and contact police with concerns.
Police are warning owners of holiday lets to beware of sex workers using their properties as pop-up brothels.