The Isle of Purbeck is a peninsula in Dorset, England. It is bordered by water on three sides: the English Channel to the south and east, where steep cliffs fall to the sea; and by the marshy lands of the River Frome and Poole Harbour to the north. Its western boundary is less well defined, with some medieval sources placing it at Flower's Barrow above Worbarrow Bay. John Hutchins, author of The History and Antiquities of the County of Dorset, defined Purbeck's western boundary as the Luckford Lake stream, which runs south from the Frome. According to writer and broadcaster Ralph Wightman, Purbeck "is only an island if you accept the barren heaths between Arish Mell and Wareham as cutting off this corner of Dorset as effectively as the sea." The most southerly point is St Alban's Head.

Dorset 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.