Carmarthenshire (Welsh: Sir Gaerfyrddin) is a one of thirteen historic counties and a principal area in Wales. Its main towns are Llanelli, Carmarthen and Ammanford. Carmarthenshire is located in West Wales.

The county is bounded to the north by Ceredigion/Cardiganshire, to the east by Powys/Brecknockshire and West Glamorgan, to the south by the Bristol Channel and to the west by Pembrokeshire. Carmarthenshire has a population of approximately 170,000, 50.3% of whom are Welsh speakers. The surface generally is upland and mountainous. Fforest Fawr and Black Mountain extend into the east of the county and the Cambrian Mountains into the north. The south coast contains many fishing villages and sandy beaches. The highest point is Carmarthen Fan, 2,525 feet (770 m). Carmarthenshire is the largest historic county by area in Wales.

Carmarthenshire became an administrative county with a county council taking over functions from the Quarter Sessions under the Local Government Act 1888. Under the Local Government Act 1972, the administrative county of Carmarthenshire was abolished on April 1, 1974, and the area of Carmarthenshire became three districts within the new county of Dyfed : Carmarthen, Dinefwr and Llanelli. Under the Local Government (Wales) Act 1994, Dyfed was abolished on April 1, 1996, and the three districts united to form a unitary authority.