The Duke of Lancaster's Regiment (King's, Lancashire and Border) (LANCS) is an infantry regiment of the line within the British Army, part of the King's Division. Headquartered in Preston, it recruits throughout the North West of England. The title of Duke of Lancaster merged with the Crown on the accession of Henry V in 1413 and remains dormant, subject to any future revival. Customarily, however, the Sovereign (whether male or female) is referred to as the Duke of Lancaster within Lancashire and in relation to the Duchy of Lancaster, and is the regiment's Colonel in Chief. The Duke of Lancaster's Regiment is the county regiment for Cumbria, Lancashire, Greater Manchester, Merseyside and the Isle of Man, and as such, recruits mainly from these areas.

The regiment's formation was announced on 16 December 2004 by Geoff Hoon and General Sir Mike Jackson as part of the restructuring of the infantry, when it was initially to be known as the King's Lancashire and Border Regiment. The regiment was given its new name in November 2005. Initially formed of three regular army battalions, it was eventually reduced to two regular battalions, plus an Army Reserve battalion. The regiment was formed through the merger of three single battalion regiments:

The regiment was formed on 1 July 2006. Initially, on formation, the regiment contained three regular battalions, with each battalion simply being renamed:

In March 2007, the 3rd Battalion was disbanded, with its personnel dispersed to the other two, leaving the final roll of two regular battalions and one Reserve battalion.

In 2012, a Kenyan mother and alleged prostitute called Agnes Wanjiru was allegedly murdered by soldiers of the Duke of Lancaster Regiment, with the murder reportedly covered up by British Army officials. The Sunday Times said a fellow soldier had gone to the “proper people” immediately after hearing a squaddie confess to the killing, but said he was told to “shut up” when he reported it and no action was taken. Ms Wanjiru's body was not discovered until two months later, after the regiment had returned to the UK. Defence Secretary Ben Wallace denied that the British military had covered-up Wanjiru's murder, but admitted guilt to and expressed concern over the Army hierarchy turning a "blind eye" to the use of prostitutes by personnel, especially within “countries in poverty”. In 2021, screenshots of a private Facebook group chat allegedly run by soldiers of the Duke of Lancaster's Regiment were published across British newspapers. The screenshots reportedly showed the regiment's soldiers joking and laughing at memes mocking the murder of Agnes Wanjiru. The leaking of the secret chats apparently mocking the murder sparked a reinvestigation into the case by Kenyan police. In 2022, UK armed forces introduced a ban on the use of sex workers abroad for the first time. Personnel found to have engaged in what the Ministry of Defence describes as “transactional sex” face the prospect of dismissal – and they could also be prosecuted if in countries where prostitution is illegal.

In December 2021, the regiment's 2nd Battalion re-subordinated to the Ranger Regiment, as its 3rd Battalion.

In 2021 a colour sergeant from 4th Battalion, the Duke of Lancaster's Regiment was fined and dismissed for lifting the skirt of a Kenyan woman, whom he knew, in a shopping mall in Nanyuki while intoxicated. A British Army spokesperson described the colour sergeant's conduct as disgraceful and said;

"This was appalling behaviour from the senior non-commissioned officer."