Toronto Harbour or Toronto Bay is a natural bay on the north shore of Lake Ontario, in Toronto, Ontario, Canada. Today, the harbour is used primarily for recreational boating, including personal vessels and pleasure boats providing scenic or party cruises. Ferries travel from docks on the mainland to the Islands, and cargo ships deliver aggregates and raw sugar to industries located in the harbour. Historically, the harbour has been used for military vessels, passenger traffic and cargo traffic. Waterfront uses include residential, recreational, cultural, commercial and industrial sites.

There are two harbours: the original natural harbour, today named the "Inner Harbour", and the "Outer Harbour". Access into the Inner Harbour is made via either the Western Gap or Eastern Gap. The Don River drains into the Inner Harbour from the north-east, through the Keating Channel.

The makeup of the soil between the mainland and the island varies depending on the area of the harbour. Near the Western Gap (the western access point to Toronto's Inner Harbour), the sediment is made up of stone, whereas sand makes up the sediment near Billy Bishop Island Airport, and the western parts of the Toronto Islands' north shore. Clay is more prominent in near the centre of the harbour, whereas the soil turns to mud near the north shore, towards the mouth of the Don River.^{[citation needed]}

The Inner Harbour is used by pleasure boats and commercial vessels. The Port of Toronto is operated by PortsToronto and is located on the eastern shore of the harbour. The port consists of cargo facilities and the International Marine Passenger Terminal, a passenger ship dock on the eastern shore. The north shore has a mixed range of uses including Harbourfront, a conversion from industrial land to recreational and cultural uses. Harbourfront has parks, hotels, an amphitheatre, and many other facilities. The north shore retains one port-related industrial use, the Redpath Sugar Refinery, while most of the lands have been converted to other uses. The Jack Layton Ferry Terminal is located at the foot of Bay Street and pleasure and party cruise boats dock along the shore to the west of York Street.

The Toronto Islands are a chain of small islands located just offshore from Downtown Toronto, providing shelter for the inner harbour. Most of the Islands is parkland, although it is also the site of several boat clubs, an amusement park, an airport, and a small residential area.

The Western Gap is a 120 metres (390 ft) wide channel allowing western access to the Inner Harbour. The gap is deep enough (over 31 metres) to allow large ships (like lake freighters) to enter and exit into the Inner Harbour. The Billy Bishop Toronto City Airport is located on the south side of the channel and is accessed by ferry and tunnel. Before the Western Gap was dug in 1910, the waterway was wide with a shallow sandy shoal surrounding what became Hanlan's Point. The sands were deposited on either side of the new channel and are part of the airport lands.

The Eastern Gap is an approximately 200 metres (660 ft) wide passage between Ward's Island and the western edge of the Port Lands and used by most freighters to enter into the inner harbour and port facilities. The gap was first formed from 1852 to 1858 when storms caused a break in the sandy spit that connected the area with the mainland.

Prior to the 1800s, small boat users had to use a portage on the western end of the sandy spit peninsula (thus requiring them to travel a short distance on land) from Lake Ontario to the inner harbour. After 1858, the Harbour Trust made the temporary channel into a permanent waterway.