A news release Wednesday from Cowessess and the Federation of Sovereign Indigenous First Nations (FSIN), which represents Saskatchewan's First Nations, did not give a specific number but said it will be the most found to date in Canada.
A press conference will be held on Thursday to reveal the details of the findings.
It comes after the remains of 215 Indigenous children were found at the site of another former Catholic-operated residential school in Kamloops, British Columbia, forcing Canadians to confront the legacy of an abusive system which forcibly separated the children from their families.
According to various estimates, from 3,200 to 6,000 children died in Canadian boarding schools for indigenous peoples, over the entire period of their existence (from the mid-19th century).
"I urge all Canadians to stand with First Nations in this extremely hard and emotional time".
The communal gravesite was first used in 1885 and later taken over by the Marieval Indian Residential School, founded in 1899 by a Catholic group on what was then the Marieval Reserve, according to the National Post.
"The first priority is the health, healing and wellness of the survivors and their families, as well as the community of Cowessess First Nation", Larissa Burnouf from FSIN Communications said.
The residential school was later demolished in 1999 and replaced with a day school.
The Marieval Indian Residential School closed in 1997 after operating for almost 100 years.
For those in need of emotional support, the 24-hour Indian Residential Schools Crisis Line is available at 1-866-925-4419.
The memorandum seeks to make records of the SSA's involvement in the school accessible to Indigenous communities, including the Tk'emlúps te Secwe̓pemc First Nation in Kamloops.