Trestman was hired as the Argonauts' 43rd head coach in February 2017 and went on to lead them to a thrilling 27-24 victory over the Calgary Stampeders in the 105th Grey Cup in Ottawa after a 9-9 regular season.
Toronto followed up its championship run by going 4-14 and finishing fourth in the East Division this season.
Argonauts general manager Jim Popp will begin the search for the team's next head coach immediately.
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