Some even brazenly declare "XXX massage" in the window.
But these days Montreal's illicit massage parlours, viewed as a haven for prostitution and even human trafficking, are coming under increased scrutiny.
Newly elected mayor Denis Coderre has made a crackdown on the parlours one of his first orders of business at city hall.
Coderre wants to introduce legislation against them that would include hefty fines.
The move isn't without detractors, however.
It's part of a larger debate about the sex trade that has made its way to the Supreme Court of Canada.
This Friday, Canada's top court will bring down a landmark decision on the legality of the country's anti-prostitution laws.
Prostitution isn't illegal in Canada, but many of the activities associated with prostitution are classified as criminal offences — including operating a brothel, living off the avails and soliciting.
The Supreme Court must decide whether to keep Canada's current prostitution laws, throw them out, or offer an alternative.
Depending on Friday's ruling, erotic massage parlours like the ones popping up all over Montreal could, conceivably, become licensed, regulated venues for the sex trade.
As it stands, the establishments often hold a permit as a legitimate therapeutic massage business, hiding their true purpose.
Montreal has an estimated 350 illicit massage parlours operating across the city and many more in its suburbs.
Other cities have already taken steps to crack down on them.
Toronto introduced a bylaw this year making a distinction between massage parlours and erotic ones, and limiting the number of licenses available for erotic ones.
Police in Calgary, meanwhile, raided a massage parlour in late October. Authorities shut it down for allegedly serving as a front for prostitution.
These days much of the focus has been on Montreal.
The city has long been seen as a hub for prostitution with a bustling red-light district. Today, many of the strip clubs in Montreal's once-seedy centre have closed down, and the street-level prostitution driven out, to make way for condos and office buildings.
But the erotic massage parlours persist across the city.
More than 70 per cent of Montreal prostitution now takes place in such venues, according to a new survey by CLES, a group that advocates against the sexual exploitation of women.
CLES supports the new mayor's efforts, said spokeswoman Eliane Legault-Roy.
"We're really happy that there's the political will to act on exploitation of women," she said.
"We just hope resources will also be put in place to help women exit prostitution."
Police have already made a string of arrests since Coderre's announcement last month. One case involved the alleged sexual exploitation of young Romanian women.
The federal government also announced last week the creation of an RCMP unit headquartered in the city to fight human-trafficking in Canada and abroad.
Emilie Laliberte, the head of a Montreal sex-trade support and advocacy group says the timing of such actions is no coincidence, with the Supreme Court ruling looming.
"It's clear they're trying to make a clear link between sex work on the one hand, and human trafficking and sexual exploitation on the other," said Laliberte, director of Stella.
In Laliberte's view, the issue isn't so simple.
Not everyone working in the sex trade is a victim of trafficking, she said. And many workers at massage parlours are worried police action could drive sex work further underground, leading to more dangerous work conditions.
"What we're saying is, 'yes' to cracking down on violence, but not on the backs of workers," Laliberte said.
She's hopeful the Supreme Court ruling in favour of legalization will usher in a new approach toward the sex trade.
France is one country considering such a step.
Its lower house in parliament passed a bill last week that would decriminalize some of the acts around prostitution in an attempt to better tackle the issue of human trafficking.
The law is intended to make it easier for trafficked women to remain in France if they get out of prostitution.
Montreal is taking a different path.
Anie Samson, the borough mayor overseeing the effort to curb erotic massage parlours, said she's hopeful the crackdown will help make city neighbourhoods safer for families.
Samson said she noticed a huge upswing in licensing applications for massage parlours in her borough at the beginning of 2013. It has since put a moratorium on further applications.
She described a triplex in her neighbourhood where there are illicit massage parlours on two of the floors, with people coming and going at all hours. A family lives on the other.
"There's not even a sign. They do all the advertising online," said Samson, mayor of Villeray–Saint-Michel–Parc-Extension.
"There are children who live around there."