Toronto police say the pedestrian was struck by a bus moving eastbound on Eglinton Avenue East, west of Birchmount Avenue, just before 4 p.m.
The woman was at one point trapped under the bus following the collision.
She was subsequently taken on an emergency run to Sunnybrook Health Sciences Centre.
Staff Sgt. Brian Bowman told CBC News that members of the traffic reconstruction team are at the scene, along with additional officers investigating the collision.
The TTC tweeted that buses on the 34 Eglinton route were diverting both ways via Birchmount Avenue, because of the collision.