The company, Mandiant, identified the People's Liberation Army's Shanghai-based Unit 61398 as the most likely driving force behind the hacking.
Mandiant said it believed the unit had carried out sustained attacks on a range of industries.
"The nature of Unit 61398's work is considered by China to be a state secret; however, we believe it engages in harmful computer network operations," Mandiant said in a report yesterday.
"It is time to acknowledge the threat is originating in China, and we wanted to do our part to arm and prepare security professionals to combat that threat effectively," it said.
The Chinese Foreign Ministry said the government firmly opposed hacking, adding that it doubted the evidence provided.
"Hacking attacks are transnational and anonymous. Determining their origins are difficult. We don't know how the evidence in this report can be tenable," a spokesman said. "Arbitrary criticism based on rudimentary data is irresponsible, unprofessional and not helpful."
The spokesman cited a Chinese study which pointed to the US as being behind hacking in China.
"Of the above mentioned internet hacking attacks, attacks originating from the US rank first."
Unit 61398 is located in Shanghai's Pudong district, China's financial and banking hub, and is staffed by perhaps thousands of people proficient in English as well as computer programming, Mandiant said in its report.
The unit had stolen "hundreds of terabytes of data from at least 141 organisations across a diverse set of industries beginning as early as 2006", it said.
Most of the victims were in the US, with smaller numbers in Canada and Britain.