Merthyr Tydfil (; Welsh: Merthyr Tudful pronounced [ˈmɛrθɪr ˈtɪdvɪl] (listen)) 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.

Noted for its industrial past, Merthyr was known as the 'Iron Capital of the World' in the early 19th century, due to the scale of its iron production. The worlds first ever railway journey happened in Merthyr in 1804, travelling 9 miles from the ironworks at Penydarren to the Glamorganshire Canal on the Merthyr Tramroad. The 1851 census found Wales to be the world's first industrialised nation, as more people were employed in industry than agriculture, with Merthyr the biggest town in Wales at that time. The Ukrainian City of Donetsk, originally 'Hughesovka', was founded by John Hughes of Merthyr in 1870, when he took iron working to the area. Iron production declined in Merthyr from 1860 on, though Merthyr's population continued to rise due to the emergence of coal mining in the area, peaking with around 81,000 people in 1911.

The area is currently known for its industrial heritage and adventure tourism. Merthyr and the surrounding areas boast the Grade-I listed Cyfarthfa Castle, the world fastest seated zip line, the UK's largest mountain bike park, the largest indoor climbing wall in Wales, national cycle routes and plans for the UK's longest indoor ski slope.