Gainsborough and Horncastle is a former county constituency represented in the House of Commons of the Parliament of the United Kingdom.

It elected one Member of Parliament (MP) by the first past the post system of election.

The area of and around Horncastle is now in the constituency of Louth and Horncastle.

For elections prior to 1983, and from 1997, see Gainsborough.

The District of West Lindsey, and the District of East Lindsey wards of Binbrook, Donington on Bain, Horncastle, Roughton, Woodhall Spa, and Wragby.