The Sydney Basin is an interim Australian bioregion and is both a structural entity and a depositional area, now preserved on the east coast of New South Wales, Australia and with some of its eastern side now subsided beneath the Tasman Sea. The basin is named for the city of Sydney, on which it is centred.

The Sydney Basin is part of a major basin system that extends over 1 500km from the Bowen Basin in Queensland through to the Gunnedah Basin in NSW. Onshore, the basin contains 4 500 metres of Permo-Triassic clastic sediments, while the offshore basin contains 6 000 metres of sediments.

The Sydney Basin is part of the Sydney–Gunnedah–Bowen Basin system, a major basin system extending from southern coastal New South Wales to central Queensland. The total onshore area of the Sydney Basin is approximately 44 000 km2 with an offshore component of about 5 000 km2.

The Sydney Basin is a major structural basin containing a thick Permian-Triassic (290 Ma - 200 Ma (million years old)) sedimentary sequence that is part of the much larger Sydney-Gunnedah-Bowen Basin.