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

From 1885 to 1983, Louth and Horncastle both existed as separate constituencies. Then in 1983, Horncastle was moved into the new seat of Gainsborough and Horncastle, while Louth was moved into the newly formed East Lindsey constituency. These boundaries remained the same until 1997, when the current Louth and Horncastle constituency was formed.

The District of East Lindsey wards of Alford, Chapel St Leonards, Coningsby, Donington on Bain, Fotherby, Grimoldby, Halton Holegate, Hogsthorpe, Holton le Clay, Horncastle, Hundleby, Legbourne, Mablethorpe, Mareham le Fen, Marshchapel, New Leake, North Holme, North Somercotes, North Thoresby, Partney, Priory, Roughton, St James', St Margaret's, St Mary's, St Michael's, Spilsby, Sutton and Trusthorpe, Tattershall, Tetford, Tetney, Theddlethorpe St Helen, Trinity, Willoughby with Sloothby, Withern with Stain, and Woodhall Spa.

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 District of East Lindsey wards of: Alford; Binbrook; Coningsby & Mareham; Fulstow; Grimoldby; Hagworthingham; Halton Holegate; Holton-le-Clay & North Thoresby; Horncastle; Legbourne; Mablethorpe; Marshchapel & Somercotes; North Holme; Priory & St. James’; Roughton; St. Margaret’s; St. Mary’s; St. Michael’s; Spilsby; Sutton on Sea; Tetford & Donington; Tetney; Trinity; Withern & Theddlethorpe; Woodhall Spa; Wragby.

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. 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.

The seat was created in 1997. Conservatives have been dominant in the area for decades, the closest result was in 1997, when a Labour Party candidate came the closest of any opponents to being elected.

The MP for this seat is Victoria Atkins. She succeeded Peter Tapsell at the 2015 general election. He previously represented the predecessor seats of East Lindsey and Horncastle from 1966 to 1997, and before that represented Nottingham West from 1959 to 1964 before being defeated by Labour. Prior to standing down, he was the longest-serving Conservative MP, albeit with the break in service, and from 2001, he was the only MP of any party first elected in the 1950s. Following the retirement of Alan Williams, Tapsell became, on his re-election in 2010, Father of the House. He was succeeded in the honorific position in 2015 by Sir Gerald Kaufman.

Before 1997, see East Lindsey

Election		Member	Party
	1997	Sir Peter Tapsell	Conservative
	2015	Victoria Atkins	Conservative

General election 2024: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Victoria Atkins	17,441	37.5	−35.0
	Reform UK	Sean Matthews	11,935	25.7	N/A
	Labour	Jonathan Slater	10,475	22.5	+4.8
	Green	Robert Watson	2,504	5.4	N/A
	Liberal Democrats	Ross Pepper	2,364	5.1	−2.7
	Independent	Paul Hugill	1,359	2.9	N/A
	Monster Raving Loony	Iconic Arty-Pole	309	0.7	−1.3
	SDP	Marcus Moorehouse	92	0.2	N/A
Majority			5,506	11.8	−43.4
Turnout			46,479	60.6	−4.9
Registered electors			76,882
	Conservative hold		Swing

General election 2019: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Victoria Atkins	38,021	72.7	+8.8
	Labour	Ellie Green	9,153	17.5	−9.2
	Liberal Democrats	Ross Pepper	4,114	7.9	+4.1
	Monster Raving Loony	The Iconic Arty-Pole	1,044	2.0	+1.1
Majority			28,868	55.2	+18.0
Turnout			52,332	65.5	−3.7
	Conservative hold		Swing	+8.95

General election 2017: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Victoria Atkins	33,733	63.9	+12.7
	Labour	Julie Speed	14,092	26.7	+8.7
	UKIP	Jonathan Noble	2,460	4.7	−16.7
	Liberal Democrats	Lisa Gabriel	1,990	3.8	−0.7
	Monster Raving Loony	The Iconic Arty Pole	496	0.9	+0.4
Majority			19,641	37.2	+7.4
Turnout			52,771	69.2	+2.0
	Conservative hold		Swing	+2.0

General election 2015: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Victoria Atkins	25,755	51.2	+1.6
	UKIP	Colin Mair	10,778	21.4	+17.1
	Labour	Matthew Brown	9,077	18.0	+0.7
	Liberal Democrats	Lisa Gabriel	2,255	4.5	−17.7
	Green	Romy Rayner	1,549	3.1	New
	Lincolnshire Independent	Daniel Simpson	659	1.3	+0.2
	Monster Raving Loony	Peter Hill	263	0.5	New
Majority			14,977	29.8	+2.4
Turnout			50,336	67.23	+2.2
	Conservative hold		Swing	-7.8

General election 2010: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Peter Tapsell	25,065	49.6	+3.2
	Liberal Democrats	Fiona Martin	11,194	22.2	+1.6
	Labour	Patrick Mountain	8,760	17.3	−8.0
	BNP	Julia Green	2,199	4.4	New
	UKIP	Pat Nurse	2,183	4.3	−3.4
	Lincolnshire Independent	Daniel Simpson	576	1.1	New
	English Democrat	Colin Mair	517	1.0	New
Majority			13,871	27.4	+5.3
Turnout			50,494	65.0	+2.8
	Conservative hold		Swing	+0.8

General election 2005: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Peter Tapsell	21,744	46.6	−1.9
	Labour	Frank Hodgkiss	11,848	25.4	−6.1
	Liberal Democrats	Fiona Martin	9,480	20.3	+0.2
	UKIP	Christopher Pain	3,611	7.7	New
Majority			9,896	21.2	+4.2
Turnout			46,683	62.0	−0.1
	Conservative hold		Swing	+2.1

General election 2001: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Peter Tapsell	21,543	48.5	+5.1
	Labour	David Bolland	13,989	31.5	+1.9
	Liberal Democrats	Fiona Martin	8,928	20.1	−4.3
Majority			7,554	17.0	+3.2
Turnout			44,460	62.1	−10.3
	Conservative hold		Swing	+1.6

General election 1997: Louth and Horncastle

Party		Candidate	Votes	%	±%
	Conservative	Peter Tapsell	21,699	43.4
	Labour	John Hough	14,799	29.6
	Liberal Democrats	Fiona Martin	12,207	24.4
	Green	Rosemary Robinson	1,248	2.5
Majority			6,900	13.8
Turnout			49,953	72.6
	Conservative win (new seat)

• Louth constituency (1885–1983)
• List of parliamentary constituencies in Lincolnshire

• Louth and Horncastle UK Parliament constituency (boundaries April 1997 – April 2010) at MapIt UK
• Louth and Horncastle UK Parliament constituency (boundaries April 2010 – May 2024) at MapIt UK
• Louth and Horncastle UK Parliament constituency (boundaries from June 2024) at MapIt UK

Parliament of the United Kingdom
Preceded by Swansea West	Constituency represented by the father of the House 2010–2015	Succeeded by Manchester Gorton

53°18′N 0°00′E / 53.30°N 0.00°E / 53.30; 0.00