The University of Hong Kong (HKU) study found the number was six times higher than four years ago, the BBC reported, adding that the increase in PTSD symptoms corresponds with around 2 million adults in the semi-autonomous territory.
Hong Kong has an adult population of about 6.3 million.
According to the study, up to 11 percent of adults reported symptoms of depression, up from two percent before separate protests in 2014 and 6.5 percent in 2017,
"One in five adults now reports probable depression or suspected PTSD, which is comparable to those experiencing armed conflicts, large-scale disasters, or terrorist attacks," the study said.
The clashes between protestors and police have led to nearly 7,000 arrests, 2,500 injuries and several deaths, according to reports.
The researchers also found that heavy use of social media to follow socio-political events appeared to increase the risk of probable depression and suspected PTSD, Agence France-Presse reported.
"Hong Kong is under-resourced to deal with this excess mental health burden," said research leader Gabriel Leung, HKU's dean of medicine, according to AFP.
The researchers, who published their findings Friday in The Lancet medical journal, conducted surveys of more than 18,000 Hong Kong residents between 2009 and 2019, according to the BBC.