The Ontario Provincial Police were called to the beach at Long Point Provincial Park at 4:10 p.m.
The man had slipped below the surface of the water and family and friends were unable to find him.
About 60 people at the beach took part in a human chain in an effort to locate the missing man.
He was found about 25 minutes later.
When pulled from the water, paramedics were unable to revive the man and he was pronounced dead at the scene.
Police say the victim is a 29-year-old man from Etobicoke. His name has not yet been released.
A post-mortem examination is scheduled to take place on Tuesday.