RCMP say the man was fishing at the Bright Water Reservoir near Hanley, just south of Saskatoon, on Sunday when he pulled an object out of the water.
The fisherman thought it might be a bomb.
The RCMP explosives disposal unit confirmed it was some type of homemade explosive device and Cpl. Rob King says it was defused at the scene.
King would not say how big the bomb was.
Police are trying to find out how it got in the reservoir.