Aberdare (/ˌæbərˈdɛər/ ab-ər-DAIR; Welsh: Aberdâr ^ⓘ) is a town in the Cynon Valley area of Rhondda Cynon Taf, Wales, at the confluence of the Rivers Dare (Dâr) and Cynon. Aberdare has a population of 39,550 (mid-2017 estimate). Aberdare is 4 miles (6 km) south-west of Merthyr Tydfil, 20 miles (32 km) north-west of Cardiff and 22 miles (35 km) east-north-east of Swansea. During the 19th century it became a thriving industrial settlement, which was also notable for the vitality of its cultural life and as an important publishing centre.

The name Aberdare means "mouth/confluence of the river Dare", as the town is located where the Dare river (Welsh: Afon Dâr) meets the Cynon (Welsh: Afon Cynon). While the town's Welsh spelling uses formal conventions, the English spelling of the name reflects the town's pronunciation in the local Gwenhwyseg dialect of South East Wales.

Dâr is an archaic Welsh word for oaks (derwen is the singulative), and the valley was noted for its large and fine oaks as late as the 19th century. In ancient times, the river may have been associated with Daron, an ancient Celtic goddess of oak. As such, the town would share an etymology with Aberdaron and the Daron river. As with many Welsh toponyms, it is likely that the locality was known by this name long before the development of the town.

There are several cairns and the remains of a circular British encampment on the mountain between Aberdare and Merthyr. This may have led to the mountain itself being named Bryn-y-Beddau (hill of graves) although other local traditions associate the name with the Battle of Hirwaun Wrgant.

Aberdare lies within the commote (cwmwd) of Meisgyn, in the cantref of Penychen. The area is traditionally given as the scene of the battle of Hirwaun Wrgant, where the allied forces of the Norman Robert Fitzhamon and Iestyn ap Gwrgant, the last Welsh prince of Glamorgan, defeated Rhys ap Tewdwr, prince of Dyfed. The battle is thought to have started at Aberdare, with the areas now known as Upper and Lower Gadlys (The battle Court(s)), traditionally given as each armies' headquarters.

The settlement of Aberdare dates from at least this period, with the first known reference being in a monastic chapter^{[clarification needed]} of 1203 concerning grazing right on Hirwaun Common. It was originally a small village in an agricultural district, centred around the Church of St John the Baptist, said to date from at least 1189. By the middle of the 15th century, Aberdare contained a water mill in addition to a number of thatched cottages, of which no evidence remains.

Aberdare grew rapidly in the early 19th century through two major industries: first iron, then coal. A branch of the Glamorganshire Canal (1811) was opened to transport these products; then the railway became the main means of transport to the South Wales coast. From the 1870s onwards, the economy of the town was dominated by the coal mining industry, with only a small tinplate works. There were also several brickworks and breweries. During the latter half of the 19th century, considerable improvements were made to the town, which became a pleasant place to live, despite the nearby collieries. A postgraduate theological college opened in connection with the Church of England in 1892, but in 1907 it moved to Llandaff.

With the ecclesiastical parishes of St Fagan's (Trecynon) and Aberaman carved out of the ancient parish, Aberdare had 12 Anglican churches and one Catholic church, built in 1866 in Monk Street near the site of a cell attached to Penrhys monastery; and at one time there were over 50 Nonconformist chapels (including those in surrounding settlements such as Cwmaman and Llwydcoed). The services in the majority of the chapels were in Welsh. Most of these chapels have now closed, with many converted to other uses. The former urban district included what were once the separate villages of Aberaman, Abernant, Cwmaman, Cwmbach, Cwmdare, Llwydcoed, Penywaun and Trecynon.