Toronto police responded to a call about a distressed person at 26 Harbour Sq on Thursday afternoon, according to a statement by the SIU.
The 45-year-old man fell from the 26th floor to a seventh floor terrace at about 1:30 p.m. He died on the scene.
It is unclear what time police arrived at the downtown apartment building.
Five SIU investigators and two forensic investigators are now looking into the incident.
The SIU conducts independent investigations whenever police officers are involved in situations when someone dies, has been seriously injured or alleges sexual assault. The SIU determines whether a criminal offence took place.