Old Sarum was from 1295 to 1832 a parliamentary constituency of England, of Great Britain, and finally of the United Kingdom of Great Britain and Ireland. It was a so-called rotten borough, with an extremely small electorate that was consequently vastly over-represented and could be used by a patron to gain undue influence. The constituency was on the site of what had been the original settlement of Salisbury, known as Old Sarum. The population and cathedral city had moved in the 14th century to New Sarum, at the foot of the Old Sarum hill. The constituency was abolished under the Reform Act 1832.

A rotten or pocket borough, also known as a nomination borough or proprietorial borough, was a parliamentary borough or constituency in England, Great Britain, or the United Kingdom before the Reform Act 1832, which had a very small electorate and could be used by a patron to gain unrepresentative influence within the unreformed House of Commons. The same terms were used for similar boroughs represented in the 18th-century Parliament of Ireland. The Reform Act 1832 abolished the majority of these rotten and pocket boroughs.