​SOUTHSEA, a seaside resort of Hampshire, England, part of the municipal and parliamentary borough of Portsmouth, with a terminal station (East Southsea) on a branch of the London & South-Western and London, Brighton & South Coast railways. It forms the southern and residential quarter of Portsmouth, and overlooks Spithead, the inlet of the English Channel between the Isle of Wight and the mainland on the north-east. There are two piers, and a parade along the sea-wall; and the sea-bathing is good. Southsea Castle was built by Henry VIII. at the southern extremity of Portsea Island. (See Portsmouth.)