Cuckmere Haven (also known as the Cuckmere estuary) is an area of flood plains in Sussex, England, where the river Cuckmere meets the English Channel between Eastbourne and Seaford. The river is an example of a meandering river, and contains several oxbow lakes. It is a popular tourist destination with an estimated 350,000 visitors per year, where they can engage in long walks, or water activities on the river. The beach at Cuckmere Haven is next to the famous chalk cliffs, the Seven Sisters.

The wreck of the Polynesia, a German sailing ship that ran aground in April 1890 west of Beachy Head laden with a cargo of sodium nitrate, is exposed at low tide.

The coastguard cottages at Cuckmere Haven were built shortly after the National Coastguard Service was founded in 1822. Prior to this Cuckmere Haven was an important place for smuggling. The cliff face in front of the cottages has since retreated by more than 30 metres due to coastal erosion. This combined with the loss of shingle from the beach below the cottages due to wave action in storms has put the cottages in danger. A fundraising campaign has been created to protect these iconic buildings

Cuckmere Haven is home to a large variety of wildlife, and has a rich ecosystem. Sheep and cattle graze, keeping the fields at a constant vegetation level. Woodpigeons and rooks pick at the stubble, and aquatic birds such as oystercatchers forage in the water meadows. The result is a fertile area of land providing varied habitats. The beaches are made up of shingle and there are rock pools along the shoreline at low tide.

From 2019, the entire coastline of the South Downs National Park is a Marine Conservation Zone.