Most of Dorset's landscape falls into two categories, determined by the underlying geology. There are a number of large ridges of limestone downland, much of which have been cleared of the native forest and are mostly grassland and some arable agriculture. These limestone areas include a band of chalk which crosses the county from south-west to north-east incorporating Cranborne Chase, the Dorset Downs and Purbeck Hills. Between the areas of downland are large, wide clay vales (primarily Oxford Clay with some Weald Clay and London Clay) with wide flood plains. These vales are primarily used for dairy agriculture, dotted with small villages, farms and coppices. They include the Blackmore Vale (Stour valley) and Frome valley.

South-east Dorset, around Poole and Bournemouth, lies on very non-resistant Eocene clays (mainly London Clay and Gault Clay), sands and gravels. These thin soils support a heathland habitat which supports all seven native British reptile species. The River Frome estuary runs through this weak rock, and its many tributaries have carved out a wide estuary. At the mouth of the estuary sand spits have been deposited turning the estuary into Poole Harbour, one of several worldwide which claim to be the second largest natural harbour in the world (after Sydney Harbour, though Sydney's claim is disputed). The harbour is very shallow in places and contains a number of islands, notably Brownsea Island, famous for its Red Squirrel sanctuary and as the birthplace of the Scouting movement. The harbour, and the chalk and limestone hills of the Purbecks to the south, lie atop Britain's largest onshore oil field. The field, operated by BP from Wytch Farm, produces a high-quality oil and boasts the world's oldest continuously pumping well (Kimmeridge, since the early 1960s) and longest horizontal drill (8 km/5 mi, ending underneath Bournemouth pier). The pottery produced by Poole Pottery from the local clays is famous for its quality.

Most of Dorset's coastline was designated a World Heritage Site in 2001 because of its geological landforms. The coast documents the entire Mesozoic era from Triassic to Cretaceous, and has yielded many important fossils, including the first complete Ichthyosaur and fossilised Jurassic trees. The coast also features examples of most notable coastal landforms, including a textbook example of cove (Lulworth Cove) and natural arch (Durdle Door). Jutting out into the English Channel is a limestone island, the Isle of Portland, connected to the mainland by Chesil Beach, a tombolo.

In the west of the county the chalk and clay of south-east England begins to give way to the marl and granite of neighbouring Devon. Until recently Pilsdon Pen at 277 metres (909 ft), was thought to be the highest hill in Dorset, but recent surveys have shown nearby Lewesdon Hill to be higher, at 279 metres (915 ft). Lewesdon is also a Marilyn.

The county has the highest proportion of conservation areas in England including an Area of Outstanding Natural Beauty (44% of the whole county), a World Heritage Site (114 km/71 mi),[3] two Heritage Coasts (92 km/57 mi)[3] and Sites of Special Scientific interest (199.45 kmē/49,285 acres).The South West Coast Path, a National Trail, runs along the Dorset coast from the Devon boundary to South Haven Point near Poole.