Louth and Horncastle is a constituency[n 1] in Lincolnshire represented in the House of Commons of the UK Parliament since 2015 by Victoria Atkins, a Conservative.

The District of East Lindsey wards of Alford, Binbrook, Chapel St Leonards, Coningsby and Tattershall, Grimoldby, Halton Holegate, Holton le Clay, Horncastle, Hundleby, Legbourne, Ludford, Mablethorpe Central, Mablethorpe East, Mablethorpe North, Mareham le Fen, Marshchapel, North Holme, North Somercotes, North Thoresby, Priory, Roughton, St James', St Margaret's, St Mary's, St Michael's, Skidbrooke with Saltfleet Haven, Spilsby, Sutton on Sea North, Sutton on Sea South, Tetford, Tetney, Trinity, Trusthorpe and Mablethorpe South, Willoughby with Sloothby, Withern with Stain, and Woodhall Spa.

Further to the 2023 Periodic Review of Westminster constituencies, which came into effect for the 2024 general election, the composition of the constituency is as follows (as they existed on 1 December 2020): The two wards of Chapel St Leonards and Willoughby with Sloothby were transferred to Boston and Skegness, whilst the previously orphaned ward of Wragby was added from Gainsborough.

This large stretch of Lincolnshire coastline includes the seaside resorts of Mablethorpe and Sutton-on-Sea, this immediate coastline has a minority of small output areas with severe deprivation following the decline of the tourism industry apart from holiday cottages, being further north than popular family resorts in Skegness.

[4] The seat's eponymous towns of Louth and Horncastle lie inland amid a rural area of the Lincolnshire Wolds where farming is an important sector, with most constituents on modest to mid-level income and low unemployment.