Cleator Moor /ˈkliːtər ˈmʊər/ is a town and civil parish in the Cumberland district of Cumbria, England. The parish was historically called Cleator. During the Industrial Revolution in the 19th century, the new settlement of Cleator Moor was built on the moorland to the north of Cleator village, based around mining and the iron industry. There was a significant influx of Irish immigrants in the latter half of the 19th century, leading to the colloquial title of Little Ireland. Cleator Moor grew to become the main settlement in the parish, which was formally renamed Cleator Moor in 1934. Cleator village remains part of the parish, and is now classed as part of the built up area of Cleator Moor by the Office for National Statistics.

The town stands below Dent Fell, and is on the 190 miles (310 km) Coast to Coast Walk that spans Northern England.

The name Cleator derives from the Old English word clāte meaning 'burdock' and the Old Norse erg meaning 'shieling'.

The town grew up to serve the iron works industry. The town had several iron ore mines and excessive mining caused subsidence. Some parts of the town have been demolished due to undermining in the area, most notably the whole of Montreal Street including the original Montreal Primary School.

The iron works was served by two railways. The Whitehaven, Cleator and Egremont Railway (WC&ER) was the first, opening for goods traffic in 1855, then two years later for passenger traffic. The WC&ER sold out to the London and North Western Railway in 1878 but when the Furness Railway objected to the sale it too became a partner, thus forming the Furness & London and North Western Joint Railway the following year. The second railway to serve Cleator Moor was the Cleator & Workington Junction Railway. This new company had a station on the western edge of the town and its double track main line made a junction with the former company at Cleator Moor West. The Whitehaven, Cleator and Egremont Railway suffered from subsidence which forced it to build two deviation branch lines and stations. In Cleator Moor itself a new line was built curving further northwest than the original, with a new station being opened in 1866 some 600 yards further west along Leconfield Street than the original, which became a goods station. The new station was known simply as Cleator Moor, but was renamed Cleator Moor East in 1924. Subsidence also necessitated a deviation at Eskett. As in Cleator Moor itself, a new line was built to the west of the original Eskett station which was retained as a goods station up to 1931. Yeathouse station was opened on the deviation line as a replacement.

The influx of Irish workers gave the town the nickname Little Ireland. As well as the settled Irish community, World War I and World War II saw an influx of immigrants from mainland Europe. In 1938, Jakob Spreiregen founded the company Kangol in Cleator, situated across the road from St Mary's Church. The original factory building still stands but is empty, since the company ended its association with the town in 2009. With the decline of traditional industries and the resulting high rate of unemployment, the town's economy is now dependent on the nearby Sellafield complex, which provides jobs to around half the town's people.

It may be that the Irish Famine prompted some increased migration to the town, but links between West Cumbria and the northern counties of Ireland had been established before that time. Labourers crossed to work the harvest and, more permanently, to take jobs in the mines and ports long before the Famine. They were often prompted by the constant sub-division in Ireland of farmland among children. From the 1850s to the 1880s, the population expanded rapidly as rich veins of haematite (iron ore) were exploited. From a settlement of 763 in 1841, Cleator Moor grew to house 10,420 people by 1871, of whom 36% were Irish. As Donald MacRaild writes, "...formative economic developments, urban growth and the mass arrival of the Irish, took place entirely in years beyond the Famine." The Irish in Cleator Moor were predominantly Roman Catholic, but the general influx into the mines and industry of West Cumbria also brought Protestants from Ireland and with them a particular sectarianism to add to the anti-Catholicism of Victorian England.

During the late 1860s the Irish Protestant preacher William Murphy led anti-Catholic meetings throughout the country, inciting people to form mobs to attack Catholic targets. Near Chelmsford in Essex, they burnt down a Catholic convent. In May 1868, two chapels, a school, and over 100 houses and shops in Ashton-under-Lyme were ransacked. This led to the Catholic populations defending themselves and their buildings. When Murphy visited Whitehaven in April 1871, the Catholic iron ore miners of Cleator Moor were determined to confront him. The local authorities requested Murphy and his Orange Order backers to cancel his talks but they would not. He was heckled and threatened at the first meeting in the Oddfellows Hall, Whitehaven and eventually had to be escorted from the place. The following evening there was more concerted opposition as 200–300 Cleator Moor miners marched to the Hall and assaulted Murphy before the meeting began. Five men were sentenced for the attack. Murphy died in March 1872 and his death was attributed to the injuries he had received in Whitehaven.