Britain's fourth largest union is starting a campaign to recruit more lap-dancers, porn industry workers and prostitutes as members.
The GMB wants to de-stigmatise sex work
Wednesday's launch has been timed to coincide with memorials across the world for prostitutes murdered by a serial killer in America.
The GMB's entertainment and sex workers branch currently has just 200 members.
Branch national organiser Martin Smith told BBC News they were some of the most vulnerable workers in the country.
An estimated 250,000 work in the sex industry in Britain.
The GMB is calling for a review of laws that it says discriminate against sex workers.
The union's London branch official, Ana Lopes, is employed as an adult chat line worker.
Strippers are among those fighting for better rights
She said Britain's laws on prostitution were not clear cut - selling sex was legal but soliciting for trade was illegal.
The laws made prostitutes rely on pimps for protection, increasing the risk of violence, Ms Lopes added.
The GMB, which voted during the summer to admit sex workers, is also demanding full employment rights for sex workers, a campaign to de-stigmatise the industry and a national debate on how social policy and legislation should be applied
"We're trying to remove the stigma against sex work and sex workers," Ms Lopes said.
"We think changing the law - decriminalising sex work - is one of the steps towards it and it's a very important one."
Reaction to the idea of unionising the sex trade is mixed.
One prostitute in London told BBC correspondent Stephen Cape: "I think it would have good benefits in getting rid of pimps and addressing the drug problem."
But a pimp he spoke to thought the business was working as it was.
"I always look after them (prostitutes). If she gets into trouble I take care of it so I don't think anything can be done through unions."
At one exotic dancing club in Bristol, the GMB already has an agreement with the girls and club owners.
Club Creme spokeswoman Viv Kent told BBC Radio 4's Today programme: "It's very important that the industry is regulated. We have no problem with the unions and we're glad to have them on board and supporting us as an industry."
The GMB wants to get involved in the Home Office review of the sex industry next year.
On Wednesday, it was delivering a letter to the Solicitor General Harriet Harman, calling for a review of the case of a sex worker who claims to have been raped by a man currently serving three life sentences for the murders of prostitutes in London.
The union's Martin Smith said it was not going to be easy for sex workers to unionise because of fear of how people who run the industry would react.
"What we're saying to people is the way to overcome that fear is to come together to support each other under the flag of the union," he said.