Merthyr Tydfil (;Welsh: Merthyr Tudful [ˈmɛrθɪr ˈtɪdvɪl]) is the main town in Merthyr Tydfil County Borough. The town is administered by Merthyr Tydfil County Borough Council and is about 23 miles (37 km) north of Cardiff, Wales. The town is often known as Merthyr.
The town is said to be named after Tydfil, a daughter of King Brychan of Brycheiniog. According to her legend, she was slain at Merthyr by pagans around 480 AD; the place was subsequently named Merthyr Tydfil in her honour. Although merthyr generally means “martyr” in modern Welsh, the meaning here is closer to the Latin martyrium: the mausoleum or church built over the relics of a martyr. Similar examples, all from south Wales, are Merthyr Cynog, Merthyr Dyfan and Merthyr Mawr.