About 20 investigators executed search warrants, seeking tax-related documents, said Stephane Dion, a spokesman for the province's revenue agency.
"The investigation has been going on for several months," he told reporters on the street outside Uber's office in Old Montreal.
"The goal is to obtain as much information as possible and these documents will then be analyzed. After that, we'll be able to determine whether charges will be laid.
"Revenu Quebec is responsible for applying tax laws. We have the power to recommend that charges be laid. And they can be accompanied by fines and prison sentences of up to five years if individuals or the company are found guilty."
Dion refused to say whether any individuals were being targeted.
Asked for reaction, Uber spokeswoman Susie Heath said in an email that "we look forward to continuing to work with all stakeholders in Quebec to foster innovation, create jobs, and provide consumers with much needed affordable transit options."
She declined a request for an interview.
The raids were conducted just over two weeks after the City of Montreal announced it had seized 40 vehicles since the beginning of the year.
Uber's UberX service uses a smartphone app that links clients to drivers in privately owned vehicles, without a taxi licence, to provide rides that cost less than cab fares.
Montreal Mayor Denis Coderre stressed on Thursday it's not a question of using the technology — "it's a question of illegal transport."
"If you give someone a lift and you have money, you call that a taxi," he said. "There are permits for that and there are ways of doing it."
He noted the same problem with Uber is happening in other cities like Edmonton.
Coderre said UberX users must understand there are responsibilities when it comes to insurance and paying the taxman.
There's also the question of screening drivers.
He said that's being done by the Montreal taxi bureau, which oversees the operation of cabs in the city.
"If it's illegal transport, there are consequences and the consequences are the seizure of a vehicle," Coderre said.
— With files from Lia Levesque