Sydney Opera House and Harbour Bridge located on Port Jackson

Port Jackson is the natural harbour of Sydney, Australia, also known as Sydney Harbour and is the largest natural harbour in the world. Its best known features are the Sydney Opera House and Sydney Harbour Bridge.

The harbour was discovered and named by Lt. James Cook in 1770, although he did not enter it. It is named after Sir George Jackson, Judge Advocate of the Fleet at the time. Cook noted "there appears to be a good anchorage" in his log. Captain Arthur Phillip established the first colony in Australia at Sydney Cove inside Port Jackson in 1788. That colony grew to become Sydney.

Geologically, Port Jackson is a drowned river valley. It is 19 km long with an area of 55 sq km. It is one of three arms extending from the single entrance. North Harbour is the shortest, and is really just a large bay extending to Manly. Middle Harbour extends to the north-west. It is bridged at The Spit and Roseville. Its headwaters lie in Garigal National Park. The longest arm, officially known as Port Jackson, extends west as far as Balmain, where it is fed by the estuaries of the Parramatta and Lane Cove rivers. Port Jackson is bridged by the Sydney Harbour Bridge. It is heavily embayed. The bays on the south side tend to be wide and rounded, whereas those on the north side are generally narrow inlets. Sydney's central business district is located on Sydney Cove, a small bay on the south side. There are four extant islands within the harbour. Several other islands have been linked to the shore by land reclamation.