The Lancashire dialect (or colloquially, Lanky) refers to the Northern English vernacular speech of the English county of Lancashire. The region is notable for its tradition of poetry written in the dialect.

Lancashire emerged during the Industrial Revolution as a major commercial and industrial region. The county encompassed several hundred mill towns and collieries and by the 1830s, approximately 85% of all cotton manufactured worldwide was processed in Lancashire. It was during this period that most writing in and about the dialect took place, when Lancashire covered a much larger area than it does today (at least from an administrative point of view—the historic county boundary remains unchanged). The administrative county was subject to significant boundary changes in 1974, which removed Liverpool and Manchester with most of their surrounding conurbations to form part of the metropolitan counties of Merseyside and Greater Manchester. At this time, the detached Furness Peninsula and Cartmel (Lancashire over the Sands) were made part of Cumbria, and the Warrington and Widnes areas became part of Cheshire.

The linguist Gerard Knowles noted that Lancashire dialect was still spoken in the city of Liverpool in 1830, before the period of mass immigration from Ireland that led the dialect of the city to change radically. Modern Liverpool speech is usually treated as a separate dialect, named Scouse. In the post-war era, migration to other towns in Merseyside, and also to the new towns created at Runcorn, Skelmersdale and Warrington, has led to an expansion in the area in which Scouse is spoken, as the next generation acquired Scouse speech habits that often displaced the traditional Lancashire or Cheshire dialects of the area.

The area transferred in 1974 to modern Cumbria, known as "Lancashire over the sands", is sometimes also covered as in scope of Cumbrian dialect: for example, The Cumbrian Dictionary of Dialect, Tradition and Folklore was written by the Barrovian William Robinson and included this area. As there was mass migration in the 19th century to Barrow-in-Furness from Ireland, Staffordshire, the Black Country, Scotland and nearby rural areas, it has (like Liverpool) developed a dialect different from the surrounding rural area.

In recent years, some have also classified the speech of Manchester as a separate Mancunian dialect, but this is a much less established distinction. Many of the dialect writers and poets in the 19th and early 20th century were from Manchester and surrounding towns.

The Lancashire dialect traditionally used rhotic pronunciation, but the accents of much of the area have become non-rhotic since the middle of the 20th century.

Alexander John Ellis, one of the first to apply phonetics to English speech, divided the county of Lancashire into four areas. Three of these four were considered North Midland in his categorisation of dialects, whereas the fourth (mostly the section that is in modern Cumbria, known as "Lancashire over the sands") was considered Northern. Dialect isoglosses in England seldom correspond to county boundaries, and an area of Lancashire could have a dialect more similar to an area of a neighbouring county than to a distant area of Lancashire.

Ellis expressly excluded the Scouse dialect of Liverpool from the areas below, although his Area 22 included some sites in modern Merseyside (e.g. Newton-le-Willows, Prescot).