Keeseekoose First Nation said the graves were found near Fort Pelly and St Phillip's residential schools, BBC reported.
It is the latest finding amid a wave that has triggered a national debate over the residential school system.
Indigenous investigations across the country have found evidence of more than 1,100 graves since last spring.
Just weeks ago, the Williams Lake First Nation announced it had found evidence of 93 unmarked graves on the grounds of St Joseph Mission, a former residential school.
These government-funded compulsory boarding schools were part of a policy meant to assimilate indigenous children and destroy indigenous cultures and languages.
Some 150,000 First Nations, Métis, and Inuit children were taken from their families during this period and placed in these schools.
Survivors had long testified that children who died at the schools were buried in unmarked plots, now being rediscovered throughout the country.
Ted Quewezance, the project leader of the Keeseekoose's search, said ground-penetrating radar technology suggested there were 42 gravesites at the grounds of Fort Pelly Residential School, and an additional 12 at St Phillip's.
At a news conference on Tuesday, Mr Quewezance said the discovery matched testimony of residential school survivors.
"It was not that they could not hear, but [that] they did not believe our survivors," he said.
RHM/PR