borough (district), administrative and historic county of Kent, southeastern England, on the south side of the Thames estuary at its mouth. Swale borough includes the Isle of Sheppey (Sheppey, Isle of), 9 miles (14 km) long and 4 miles (6 km) wide, to the north. The island is separated from the mainland (south) by The Swale, a branch of the River Medway estuary, which gives its name to the district. Land on either side of The Swale is marshland, with both salt marsh and freshwater aquatic vegetation, that extends the length of the district. Population centres on the Isle of Sheppey are concentrated on a range of low cliffs along the northern coast bordering the Thames estuary. The estuarine marshland of the mainland ascends southward into a highly fertile cultivated zone that borders on the chalk uplands of the North Downs. Agricultural produce grown there includes hops, apples, and cherries.

The heavy-industrialized ports of Queenborough-in-Sheppey and Sheerness on the Isle of Sheppey have deepwater harbours. Container facilities opened at Sheerness in 1972, and the town has regular ferry service to Vlissingen (Flushing), Neth.

Sittingbourne, the district seat on the mainland, is the site of an agricultural-research complex (founded in 1940). The former medieval port of Faversham east of Sittingbourne has a restored pre-19th-century street and has expanded its contemporary port facilities. Trade in timber, fertilizers, and petroleum products passes through the port. The British historian George Finlay (Finlay, George) (1799–1875) was born in Faversham. Area 144 square miles (374 square km). Pop. (2001) 122,801.