Merthyr Tydfil is the main town in Merthyr Tydfil County Borough, Wales, administered by Merthyr Tydfil County Borough Council. It is about 23 miles (37 km) north of Cardiff. Often called just Merthyr, it is said to be named after Tydfil, daughter of King Brychan of Brycheiniog, who according to legend was slain at Merthyr by pagans about 480 CE. Merthyr generally means "martyr" in modern Welsh, but here closer to the Latin martyrium: a place of worship built over a martyr's relics. Similar place names in south Wales are Merthyr Cynog, Merthyr Dyfan and Merthyr Mawr.

Merthyr Tydfil County Borough is a county borough in the south-east of Wales. In mid 2018, it had an estimated population of 60,183 making it the smallest local authority in Wales by both population and land area. It is located in the historic county of Glamorgan and takes its name from the town with the same name. The county borough consists of the northern part of the Taff Valley and the smaller neighbouring Taff Bargoed Valley. It borders the counties of Rhondda Cynon Taf to the west, Caerphilly County Borough to the east, and Powys to the north.