The Hay River is an 80-kilometre waterway in the Great Southern region of Western Australia, rising west of Mount Barker and flowing generally south-south-westerly through Mount Lindesay National Park before crossing the South Coast Highway and discharging into Wilson Inlet near the town of Denmark. The river is known to the traditional Noongar owners as Genulup. Its basin covers approximately 129,800 hectares, with an average discharge of around 69,400 megalitres per year.

The river flows year-round, and the water is characterised as marginally saline to brackish throughout. Tidal influence extends approximately 5 kilometres upstream from where the river meets Wilson Inlet, making the lower reaches a tidal zone that paddlers should account for when planning trips. The riparian corridor retains some natural character — roughly 42% of the riparian zone was recorded as pristine — though around 70% of the broader catchment has been cleared for agricultural use.

The lower tidal section near Denmark is accessible for upstream paddling, with at least one documented trip covering approximately 4 kilometres upstream from the estuary to a sandy beach suitable for a rest stop. The Wilson Inlet estuary, into which the Hay River flows, supports a substantial pelican population, reflecting the presence of fish in the system. The estuary's connection to the sea can be affected by sand bar formation at the inlet opening, which may influence water levels and conditions in the lower river.