Dorset ( DOR-sit; archaically: Dorsetshire DOR-sit-sheer, -⁠shər) is a ceremonial county in South West England. It is bordered by Somerset to the north-west, Wiltshire to the north and the north-east, Hampshire to the east, the Isle of Wight across the Solent to the south-east, the English Channel to the south, and Devon to the west. The largest settlement is Bournemouth, and the county town is Dorchester.

The county has an area of 2,653 km² (1,024 sq mi) and a population of 772,268. Around half of the population lives in the South East Dorset conurbation, which contains three of the county's largest settlements: Bournemouth (183,491), Poole (151,500), and Christchurch (31,372). The remainder of the county is largely rural, and its principal towns are Weymouth (53,427) and Dorchester (21,366). Dorset contains two unitary authority areas: Bournemouth, Christchurch and Poole, and Dorset. The county did not historically include Bournemouth and Christchurch, which were part of Hampshire.

Dorset has a varied landscape of chalk downs, steep limestone ridges, and low-lying clay valleys. The majority of its coastline is part of the Jurassic Coast World Heritage Site due to its geological and palaeontologic significance, and features notable landforms such as Lulworth Cove, the Isle of Portland, Chesil Beach and Durdle Door.