The SIU says around noon, Waterloo Region Police officers responded to reports of a man appearing to be in distress near Highway 85 and Northfield Drive West.
Officers "began speaking with him" and "shortly after, the man fell from the overpass to the highway below," the SIU said in a release.
The SIU is provincial agency that investigates incidents involving police where there has been death, serious injury or allegations of sexual assault.
An SIU release said officers began speaking with the 40-year-old man before he fell from the overpass onto the highway. He was pronounced dead at the scene. An autopsy is scheduled for Wednesday in Hamilton.
Waterloo Regional Police say they were called to Northfield Drive West at the bridge overlooking Highway 85 just before noon on Tuesday.
The northbound lanes of the highway were closed for several hours while emergency crews were on scene.
Police have made an arrest two months after Gerald Robert Male was fatally shot at an address on Williams Street.