The 55th Regiment of Foot was a British Army infantry regiment, raised in 1755. After 1782 it had a county designation added, becoming known as the 55th (Westmorland) Regiment of Foot. Under the Childers Reforms it amalgamated with the 34th (Cumberland) Regiment of Foot to form the Border Regiment in 1881.

The regiment was raised in Stirling by Colonel George Perry as the 57th Regiment of Foot in 1755 for service in the Seven Years' War. It was re-ranked as the 55th Regiment of Foot, following the disbandment of the existing 50th and 51st regiments, in 1756.

The regiment embarked for North America for service in the French and Indian War and arrived in Nova Scotia on 8 July 1757 with the objective of taking part in the abandoned attack on the Fortress of Louisbourg. Following the death of Colonel Perry, Lord George Augustus Viscount Howe was appointed Colonel of the regiment in September 1757. After the regiment arrived in Albany, New York in November 1757, Howe accompanied Major Robert Rogers, commander of His Majesty's Independent Companies of Rangers on a scout, to learn the art of "bush fighting." Howe's willingness to learn from the American rangers and his interaction with subordinates won him the respect of both colonist and British redcoat being described as the "Idol of the army." In the spring of 1758, Howe began to train and accoutre the men in the regiment more like rangers to better adapt them to warfare in America. He was killed in a skirmish the day before the Battle of Carillon in July 1758.

After Howe's death John Prideaux was appointed commander of the regiment. In an unfortunate accident Prideaux was killed by the blast of a cohorn while walking through the entrenchments during the Battle of Fort Niagara in July 1759. The regiment, as part of General Jeffery Amherst's army, participated in the Battle of Ticonderoga and the capture of Fort Crown Point later that month.

In 1760 Colonel James Adolphus Oughton took command of the regiment and led it up the Saint Lawrence River: the regiment witnessed the Montreal Campaign between August and September 1760. William Gansell became the colonel of the regiment in 1762.

In summer 1763, volunteers from the regiment were sent to reinforce the British post at Fort Detroit, which was under siege from neighbouring Native Americans led by Pontiac. The British force was ambushed and badly mauled en route at the Battle of Bloody Run in July 1763. In 1764 many surviving members of the regiment were drafted into the 17th Regiment of Foot. Anne Grant, whose father was an officer in the 55th Regiment, wrote; "they were going to become part of a regiment of no repute; whom they themselves had held in the utmost contempt when they formerly served together."

The regiment returned to North America for the American Revolutionary War. The regiment fought at the Battle of Long Island in August 1776 and the Battle of Princeton in January 1777. It went on to take part in the Philadelphia campaign and saw action at the Battle of Brandywine in September 1777, the Battle of Paoli later that month and the Battle of Germantown in October 1777. The regiment was transferred to the West Indies in November 1778 and saw action at the Battle of St. Lucia in December 1778. Most of the regiment were captured at the siege of Brimstone Hill in February 1782 during the French invasion of Saint Kitts. The regiment adopted a county designation as the 55th (the Westmoreland) Regiment of Foot in August 1782.

In 1793 the regiment embarked for Flanders for service in the French Revolutionary Wars and saw action at the siege of Ypres in June 1794. It then moved to the West Indies and took part in the attack on Martinique in February 1794, on Saint Lucia in April 1794 and on Guadeloupe later that month as well as the capture of Saint Lucia in May 1796. It also helped suppress an insurrection by caribs on Saint Vincent in June 1796. After returning to England in 1797 the regiment landed at Ostend in 1798 for service in the Anglo-Russian invasion of Holland. It saw action at the Battle of Bergen in September 1799 and the Battle of Alkmaar in October 1799. After returning home in 1800 the regiment was deployed to the West Indies again in 1800 and went to the aid of Britain's new found Spanish allies during the Spanish reconquest of Santo Domingo in July 1809. The regiment returned home in 1812 and, having been sent to Holland in 1813, took part in the siege of Bergen op Zoom in March 1814.