Shelter Scotland said the sector was becoming filled with "cowboy" operators because agents needed no professional expertise or experience to set up.
Complaints to the property ombudsman have risen by 123% in five years.
The Scottish government said it would publish a strategy for the private rented sector by the end of the month.
Because of the slump in the private housing market and the general economic situation more people are renting their home rather than buying.
In the past 10 years the number of households renting privately in Scotland has almost doubled to 290,000.
And there has also been an increase in the number of letting agents to about 500.
Shelter said regulation was vital as the UK government's changes to Housing Benefit was likely to increase demand for rented accommodation.'Wild west territory'
But according to the charity's director, Graeme Brown, agents are unregulated, and complaints against them are increasing.
"They're letting properties that are in disrepair; they're claiming some properties are furnished when they're unfurnished," he said.
"Some of them are setting up business, taking deposits, closing down and setting up under a different name.
"These are cowboys. This is Wild West territory and we want the Scottish government to regulate this sector."
Stacey Gowran, 23, fell victim to a rogue letting agent.
She rented a flat in Inverclyde, described on the agent's website as furnished. But as she prepared to move in, she was in for a surprise.
"There was nothing in it, save a bloodstained mattress and a broken bed frame," she said.
"When I phoned them, they said I hadn't asked for it to be furnished - even though it was advertised as a fully-furnished flat.
"The roof wasn't secure or fit-for-purpose on the building, and it affected the people above me as well.
"The windows weren't watertight; everything was an absolute mess - there was mould growing on the ceilings from all the dampness."Trading standards
It took a threat of legal action before Stacey was able to regain her deposit and move out.
"Some of them are setting up business, taking deposits, closing down and setting up under a different name.
"These are cowboys. This is Wild West territory and we want the Scottish government to regulate this sector."
Stacey Gowran, 23, fell victim to a rogue letting agent.
She rented a flat in Inverclyde, described on the agent's website as furnished. But as she prepared to move in, she was in for a surprise.
"There was nothing in it, save a bloodstained mattress and a broken bed frame," she said.
"When I phoned them, they said I hadn't asked for it to be furnished - even though it was advertised as a fully-furnished flat.
"The roof wasn't secure or fit-for-purpose on the building, and it affected the people above me as well.
"The windows weren't watertight; everything was an absolute mess - there was mould growing on the ceilings from all the dampness."Trading standards
It took a threat of legal action before Stacey was able to regain her deposit and move out.
Stuart MacKenzie has also been the victim of a rogue letting agent, having put his flat in Glasgow up for rent after his job moved to Edinburgh.
Rent was paid late, repairs charged with no receipts and a new tenant given a lease without his agreement - when he was trying to sell.
And then the agent closed her business, leaving Stuart out of pocket by several hundred pounds. He agrees the sector needs regulation.
"I would certainly back Shelter Scotland's call for regulation," he said.
"The letting agent we used had all the materials, the cars, the folders, the branding.
"But at the end of the day, I have been to the police, to trading standards, but there's very little support out there.
"And it seems there's very little to actually regulate the actions of these people."'Structured standards'
The Royal Institution of Chartered Surveyors (Rics) welcomed Shelter's calls for tighter regulation in the private rented sector.
Rics director in Scotland, Sarah Speirs, said: "The Scottish government has taken welcome steps in introducing mandatory tenancy deposit schemes to protect individual's deposits, however, Rics agrees that there is room for further regulation.
"A structured set of professional standards for letting agents is still required to make the private rented sector a more dependable option for families and individuals.
"We recommend that until these regulations are in place tenants should use a letting agent that is a member of a professional organisation, such as RICS."
Shelter Scotland said the government must ensure letting agents were effectively regulated so all tenants and landlords get a fair deal.
A spokesman for the Scottish government said it was committed to improving the situation.
"That is why we clarified the law last year to make it clear that the charging of premium payments for entering into a private rented tenancy was not allowed, and also introduced the Tenancy Deposit Scheme to safeguard tenants deposit money," he said.
The government's strategy on the private rented sector is due to be published on 30 May.
Rent was paid late, repairs charged with no receipts and a new tenant given a lease without his agreement - when he was trying to sell.
And then the agent closed her business, leaving Stuart out of pocket by several hundred pounds. He agrees the sector needs regulation.
"I would certainly back Shelter Scotland's call for regulation," he said.
"The letting agent we used had all the materials, the cars, the folders, the branding.
"But at the end of the day, I have been to the police, to trading standards, but there's very little support out there.
"And it seems there's very little to actually regulate the actions of these people."'Structured standards'
The Royal Institution of Chartered Surveyors (Rics) welcomed Shelter's calls for tighter regulation in the private rented sector.
Rics director in Scotland, Sarah Speirs, said: "The Scottish government has taken welcome steps in introducing mandatory tenancy deposit schemes to protect individual's deposits, however, Rics agrees that there is room for further regulation.
"A structured set of professional standards for letting agents is still required to make the private rented sector a more dependable option for families and individuals.
"We recommend that until these regulations are in place tenants should use a letting agent that is a member of a professional organisation, such as RICS."
Shelter Scotland said the government must ensure letting agents were effectively regulated so all tenants and landlords get a fair deal.
A spokesman for the Scottish government said it was committed to improving the situation.
"That is why we clarified the law last year to make it clear that the charging of premium payments for entering into a private rented tenancy was not allowed, and also introduced the Tenancy Deposit Scheme to safeguard tenants deposit money," he said.
The government's strategy on the private rented sector is due to be published on 30 May.
