The Borough of Dacorum is a local government district in Hertfordshire, England that includes the towns of Hemel Hempstead, Berkhamsted, Tring and the western part of Kings Langley. The district, which was formed in 1974, had a population of 137,799 in 2001. Its name was taken from the old hundred of Dacorum which covered approximately the same area.

The name Dacorum comes from Latin and it means "hundred of the Dacians". The latter word was used mistakenly in the Middle Ages for 'Danes'. This happened because of a legend asserting that certain tribes from Dacia had migrated to Denmark. The hundred of Dacorum was first recorded in 1196, although it has existed since the 9th and 10th centuries, when it lay near the southern boundary of the Danelaw, on the River Lea. In 1086, the Domesday Book records the hundreds of Tring and Danais in places that became parts of the hundred of Dacorum.

In 1974, the modern district of Dacorum was formed under the Local Government Act 1972. Major components were the municipal borough of Hemel Hempstead, the urban districts of Berkhamsted and Tring, the rural districts of Berkhamsted and Hemel Hempstead and those parts of the rural districts of St Albans and Watford which were within the designated area of Hemel Hempstead new town.

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