The Swan Coastal Plain in Western Australia is the geographic feature which contains the Swan River as it travels west to the Indian Ocean. The coastal plain continues well beyond the boundaries of the Swan River and its tributaries, as a geological and biological zone, one of Western Australia's Interim Biogeographic Regionalisation for Australia regions. It is also one of the distinct physiographic provinces of the larger West Australian Shield division.

The coastal plain is a 30-kilometre-wide (19 mi) strip on the Indian Ocean coast directly west of the Darling Scarp uplands running from Cape Naturaliste in the south to north of the city of Perth. The plain mainly consists of fairly infertile sandy soil along with coastal sand dunes, river estuaries, and a number of wetlands kept back from the sea by the dunes. A number of rivers cross the plain from east to west from the Darling Scarp towards the sea, including the Swan and its main tributary, the Canning.

The sediments of the Perth Basin are Tertiary and Quaternary in age immediately below Perth and include coquina, travertine, and sandy limestones with abundant shelly material. Perth is sited on a set of sand dunes formed during the Pliocene-Pleistocene during the last ice age. Offshore, the sand dune system and surficial deposits transition into a system of partly eroded limestones and sandy limestones. These form a series of drowned cuestas which today form submerged reefs.

Because of the steepness and orientation of the Darling Scarp, watercourses run off the scarp in a westerly direction. On reaching the Swan Coastal Plain, they encounter two obstacles: firstly, the high permeability of the soil results in the loss of much water. Much flow is lost, and some watercourses simply cease to flow, for example, Wungong Brook, Cardup Brook, and Manjedal Brook. Secondly, the sequence of north-to-south-oriented dunes create obstacles to further westerly flow. Many watercourses turn at right angles and flow south or north along the dune swales, and in some cases this allows multiple watercourses to coalesce. For example, on reaching the coastal plain, Ellen Brook runs south for around 30 kilometres (19 mi), picking up a number of watercourses along the way, before finally emptying into the Swan River. Another example is the Serpentine River, which runs west to less than 10 kilometres (6.2 mi) from the coast, then turns south, running parallel with the coast for nearly 20 kilometres (12 mi) before disgorging into Peel Inlet.

The dune topology also results in extensive north-to-south-oriented chains of wetlands, again located in the swales. For example, the chain of lakes north of Perth includes lakes of the Wanneroo wetlands including – Lake Pinjar, Jandabup Lake and Gnangara Lake. The large bodies of water in the estuary of the Swan River, Perth Water and Melville Water are also located in dune swales.

The Swan coastal plain is characterised by a series of sand dune systems, the Quindalup dunes, the Spearwood dunes, and the Bassendean dunes, which run from west to east (in increasing age) from the coastline to the major faults which form the eastern boundary of the plain. The plain is bounded to the east by the Darling Scarp, to the north by a subsidiary fault running north-west from Bullsbrook, and to the south by the Collie-Naturaliste Scarp. The Pinjarra plain lies between the Bassendean dunes and the eastern scarps. Early work on all three dune systems considered them to have been formed at differing times by the deposition of sands carried by wind (aeolian and/or by river processes (fluviatile). More recently, later authors have argued that these sandplains are mostly the product of in situ weathering.

The carbonate material of all three dune systems has been completely leached, leaving dunes which consist solely of quartz sand. Within the Spearwood dunes, the yellow or grey sand may be several metres thick.

The dune systems have been much studied, as the water retained within them supplies much of Perth's water needs. (See Gnangara Mound).