Bacup (/ˈbeɪkəp/ BAY-kəp, /ˈbeɪkʊp/) is a town in the Rossendale Borough in Lancashire, England, in the South Pennines close to Lancashire's boundaries with West Yorkshire and Greater Manchester. The town is in the Rossendale Valley and the upper Irwell Valley, 4 miles (6.4 km) east of Rawtenstall, 6 miles (9.7 km) north of Rochdale, and 7 miles (11 km) south of Burnley. At the 2011 Census, Bacup had a population of 13,323.

Bacup emerged as a settlement following the Anglo-Saxon settlement of Britain in the Early Middle Ages. For centuries, it was a small and obscure centre of domestic flannel and woollen cloth production, and many of the original weavers' cottages survive today as listed buildings. Following the Industrial Revolution, Bacup became a mill town, growing up around the now covered over bridge crossing the River Irwell and the north–south / east-west crossroad at its centre. During that time its landscape became dominated by distinctive and large rectangular woollen and cotton mills. Bacup received a charter of incorporation in 1882, giving it municipal borough status and its own elected town government, consisting of a mayor, aldermen and councillors to oversee local affairs.

In 1974, Bacup became part of the borough of Rossendale. Bacup's historic character, culture and festivities have encouraged the town to be seen as one of the best preserved mill towns in England. English Heritage has proclaimed Bacup town centre as a designated protected area for its special architectural qualities.

The name Bacup is derived from the Old English fūlbæchop. The Oxford Dictionary of British Place Names translates this as "muddy valley by a ridge"; the fūl- element, which meant "foul" or "muddy" was used in the earliest known reference to the area, in a charter by Robert de Lacey, around the year 1200, as used in the Middle English spelling fulebachope. The prefix ful- was dropped from the toponym. The -bæchop element is less clear, possibly meaning "ridge valley", or else "back valley" referring to the locale's position at the back part of the Irwell Valley.

Bacup and its hinterland has provided archeological evidence of human activity in the area during the Neolithic. Anglo-Saxons settled in the Early Middle Ages. It has been claimed that in the 10th century the Anglo-Saxons battled against Gaels and Norsemen at Broadclough, a village to the north of Bacup. From the medieval period in this area, the River Irwell separated the ancient parishes of Whalley and Rochdale (in the hundreds of Blackburn and Salford respectively). The settlement developed mainly in the Whalley township of Newchurch but extending into Rochdale's Spotland.

The geology and topography of the village lent itself to urbanisation and domestic industries; primitive weavers' cottages, coal pits and stone quarries were propelled by Bacup's natural supply of water power in the Early Modern period. The adoption of the factory system, which developed into the Industrial Revolution, enabled the transformation of Bacup from a small rural village into a mill town, populated by an influx of families attracted by Bacup's cotton mills, civic amenities and regional railway network. Locally sourced coal provided the fuel for industrial-scale quarrying, cotton spinning and shoemaking operations, stimulating the local economy. Bacup received a charter of incorporation in 1882, giving it honorific borough status and its own elected town government, consisting of a mayor, aldermen and councillors to oversee local affairs.

Bacup's boom in textile manufacture during the Industrial Revolution resulted in the town developing into a prosperous and thickly populated industrial area by early-20th century. But the Great Depression and the ensuing deindustrialisation of the United Kingdom largely eliminated Bacup's textile processing sector and economic prosperity.

Bacup followed the regional and national trend of deindustrialisation during the early and mid-20th century; a process exacerbated by the closure of Bacup railway station in 1966. Bacup also experienced population decline; from 22,000 at the time of the United Kingdom Census 1911, to 15,000 at the United Kingdom Census 1971. Much of Bacup's infrastructure became derelict owing to urban decay, despite regeneration schemes and government funding. Shops became empty and some deteriorated. The houses along the main roads endured as the original terraces from Bacup's industrial age, but behind these, on the hillsides, are several council estates.