Carmarthen ">[kɑːɨrˈvərðɪn], "Merlin's fort") is a community in, and the county town of, Carmarthenshire, Wales. It is on the River Towy 8 miles north of its mouth at Carmarthen Bay. In 2001, the population was 15,854.

Carmarthen has a strong claim to being the oldest town in Wales but the two settlements of Old and New Carmarthen were only united into a single borough in 1546. Carmarthen was the most populous borough in Wales between the 16th and 18th centuries and was described by William Camden as "the chief citie of the country". However, population growth stagnated by the mid 19th century as more dynamic economic centres developed in the South Wales coalfield. Carmarthen is the location of the headquarters of Dyfed-Powys Police, the Carmarthen campus of the University of Wales, Trinity Saint David and Glangwili General Hospital.