From west to east: Sydney Harbour[1] (Mi'kmawi'simk: L'sipuktuk) is the 10-mile long Y-shaped inlet of the Atlantic, oriented southwest-northeast on the northeast shore of Cape Breton Island, Nova Scotia.

Sydney Harbour is the maritime hub for the Cape Breton Regional Municipality.

[3] A cove on the north shore, between North Sydney and Sydney Mines, originally called Indian Cove and known locally as Barrington's Cove (after the family that owned adjacent property), was where Prince Edward (later King Edward VII) stepped ashore sometime between 28 July and 2 August 1860,[n 1] to visit the Sydney Mines while en route from St. John's, Newfoundland, to Halifax.

Muggah Creek, near the steel mill, transported material (including polychlorinated biphenyl)[10] into Sydney Harbour, resulting in the area around the mouth of the creek to have some of the most contaminated marine sediment known, as of 2003.

[11] The harbour underwent major dredging in 2011, clearing the channel to 17-meter depth and making it accessible to large container ships.