There were no immediate reports of injury or damage.
Bathurst Street was closed between Lake Shore Boulevard and Fort York Boulevard for the entire morning as crews surveyed the area and ensured the stretch was safe.
Service on the 511 Bathurst and 509 Harbourfront streetcar lines have also resumed.
John Bryson, city engineer and manager of structures and expressways, said Wednesday the extremely hot weather might be partly to blame for falling concrete.
This isn't the first time the elevated expressway has had reports of falling concrete.
On June 25 pieces fell onto a car while a man was driving to work. The driver wasn't injured, but after the incident, Toronto staff briefed a council committee on the state of the Gardiner, which they said is in need of significant rehabilitation in the next 10 years.
There have been at least five instances in just over two months of concrete falling from the underside of the Gardiner Expressway onto the road below. The map below depicts at least four other documented instances in the past few years.