Harpenden (/ˈhɑːrpəndən/) is a town and civil parish in the City and District of St Albans in the county of Hertfordshire, England. The population of the built-up area was 30,674 in the 2021 census, while the population of the civil parish was 31,128. Harpenden is a commuter town, with a direct rail connection to Central London.

There is evidence of pre-Roman Belgic farmers in the area. In 1867, several items were found, including a bronze escutcheon, rams-head shaped mounts, and a bronze bowl.

There are Roman remains in the land around Harpenden, such as the site of a mausoleum in the park at Rothamsted. A tumulus near the river Lea was opened in the 1820s, and it contained a stone sarcophagus of Romano-Celtic origin. Five objects dating from around 150 AD were inside, including a glass jug with a Mediterranean stamp and samian ware dishes used for libations.

The name Harpenden probably derives from the Old English hearpedenu, meaning 'harp valley'. Another theory suggests that the name perhaps derives from herepæðdenu meaning 'military road valley'. This theory is considered because Harpenden lies on the ancient St Albans - Luton road.

Up to the 13th century, the area of the parish consisted of woodland with small hamlets and single farmsteads around cleared areas called "End" or "Green"; today, there are 19 Ends and 18 Greens in the areas of Harpenden and Wheathampstead parishes.

Harpenden village grew out of Westminster Abbey's gradual clearing of woodland for farming and settlement within its Wheathampstead manor, granted by Edward the Confessor in 1060. The first reference to a parish church was in 1221 (when it was referred to as Harpendene) so it is inferred that the village grew up around then. The church of St Nicholas is the oldest church in the town, originally built as a Chapel of ease in 1217.

Just beyond the southern edge of the town lies Nomansland Common (sometimes simply called "No Man's Land") upon which part of the Second Battle of St Albans was fought during the Wars of the Roses. Nomansland Common also saw the first annually contested steeplechase in England, in 1830, when it was organised by Thomas Coleman, and the last fight of 19th-century bare-knuckle fighter Simon Byrne. It was also the haunt of the highwaywoman Lady Katherine Ferrers, better known as the "Wicked Lady".

A widespread but now little-known industry of Harpenden was straw-weaving, a trade mainly carried out by women in the 19th century. A good straw weaver could make as much as a field labourer. The straw plaits were taken to specialist markets in St Albans or Luton and bought by dealers to be converted into straw items such as boaters and other hats or bonnets.