The Hawkesbury River, or Hawkesbury-Nepean River (Dharug: Dyarubbin) is a river located northwest of Sydney, New South Wales, Australia. The Hawkesbury River and its associated main tributary, the Nepean River, almost encircle the metropolitan region of Sydney. The river between Wisemans Ferry and the Pacific Ocean marks the boundary of Greater Metropolitan Sydney in the south and the Central Coast region to the north.

The Hawkesbury River has its origin at the confluence of the Nepean River and the Grose River, to the north of Penrith and travels for approximately 120 kilometres (75 mi) in a north–easterly and then a south–easterly direction to its mouth at Broken Bay, about 15 kilometres (9.3 mi) from the Tasman Sea. The Hawkesbury River is the main tributary of Broken Bay. Secondary tributaries include Brisbane Water and Pittwater, which, together with the Hawkesbury River, flow into Broken Bay and thence into the Tasman Sea north of Barrenjoey Head.

The total catchment area of the river is approximately 21,624 square kilometres (8,349 sq mi) and the area is generally administered by the Hawkesbury–Nepean Catchment Management Authority.

The land adjacent to the Hawkesbury River was occupied by Aboriginal peoples: the Darkinjung, Darug, Eora, and Kuringgai. They used the river as a source of food and a place for trade. In the languages of the traditional custodians the river is Deerubben or Dyarubbin.

In March 2021, the river level rose, and many areas were flooded as part of the March 2021 Australian floods.

The headwaters of the Hawkesbury River, the Avon River, the Cataract River, and the Cordeaux River, rise only a few kilometres (miles) from the sea, about 80 kilometres (50 mi) south of Sydney. These streams start on the inland-facing slopes of the plateau which forms the escarpment behind Wollongong. Flowing north-west, away from the sea, these streams combine to form the Nepean River, and flow north past the towns of Camden and Penrith. Near Penrith, the Warragamba River emerges from its canyon through the Blue Mountains and joins the Nepean. The Warragamba, formed by the joining of the Wollondilly River, the Nattai River, the Kowmung River and Coxs River drains a broad region of New South Wales on the eastern side of the Great Dividing Range. The other principal component of the upper Hawkesbury river system, the Grose River, rises in the area of Mount Victoria in the Blue Mountains.

Once formed, the Hawkesbury River proper flows generally northwards, albeit with a significant number of meanders. Initially the river passes the towns of Richmond and Windsor, which are the largest settlements on the river. At Windsor, the river is joined by South Creek, which drains much of the urban runoff in Sydney's western suburbs that does not fall into the Parramatta River catchment. As it flows north, it enters a more rural area, with only small settlements on the river. On this stretch it passes Sackville and Lower Portland, where it is joined by the Colo River. The Colo River and its tributaries drain the northern section of the Blue Mountains.

From Lower Portland, the Hawkesbury River continues flowing northwards to the small community of Wisemans Ferry where it is joined by the Macdonald River. Here its course turns eastwards and the surrounding landscape gradually becomes steeper and more rugged. At Spencer, Mangrove Creek joins the river from the north. From here to the river mouth, road access to the river is limited to a few points.