Port Jackson is the natural harbour of Sydney, Australia. Also known as Sydney Harbour, it is the largest natural harbour in the world. The harbour was discovered and named by Lt. James Cook in 1770, although he did not enter it. It is named after the 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, of course, grew to become Sydney.