The verdict against Jacques Delisle, 77, came after three days of deliberations about the shooting death of 71-year-old Marie-Nicole Rainville in November 2009, The (Montreal) Gazette reported.
The murder conviction is thought to be the first ever for a Canadian judge, the newspaper said.
Delisle testified when he went into the couple's condo, he found his disabled wife dead on a couch with a pistol at her side and a bullet wound to the head.
The woman had a stroke two years earlier that paralyzed her right side. Months before her death, she fell and broke her hip and was in physiotherapy when she died.
Delisle told the court his wife was depressed about the loss of brain function the stroke had caused and was even more depressed after her hip surgery.
However, details emerged during the trial the former judge was having an affair with a former secretary and wanted to avoid a costly divorce from his wife, the newspaper said.
The 57-year-old secretary testified she was preparing to move in with Delisle when he was arrested in 2010.