Mid Sussex is a local government district in West Sussex, England. The largest town is Haywards Heath, where the council is based. The district also contains the towns of Burgess Hill and East Grinstead plus surrounding rural areas, including many villages. The district includes part of the South Downs National Park and part of the designated Area of Outstanding Natural Beauty of High Weald, including sections of Ashdown Forest. The district contains most headwaters of the River Ouse. Its largest body of water is Ardingly reservoir which is used by watersports clubs. At the 2021 census the district had a population of 152,949.