Carmarthenshire (/kərˈmɑːrðənʃər, -ʃɪər/ kər-MAR-dhən-shər, -⁠sheer; Welsh: Sir Gaerfyrddin [ˈsiːr ɡaːɨrˈvərðɪn] or informally Sir Gâr) is a county in the south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. The county is known as the "Garden of Wales" and is also home to the National Botanic Garden of Wales.

Carmarthenshire has been inhabited since prehistoric times. The county town was founded by the Romans, and the region was part of the Kingdom of Deheubarth in the High Middle Ages. After invasion by the Normans in the 12th and 13th centuries it was subjugated, along with other parts of Wales, by Edward I of England. There was further unrest in the early 15th century, when the Welsh rebelled under Owain Glyndŵr, and during the English Civil War.

Carmarthenshire is mainly an agricultural county, apart from the southeastern part which was once heavily industrialised with coal mining, steel-making and tin-plating. In the north of the county, the woollen industry was very important in the 18th century. The economy depends on agriculture, forestry, fishing and tourism. West Wales was identified in 2014 as the worst-performing region in the United Kingdom along with the South Wales Valleys with the decline in its industrial base, and the low profitability of the livestock sector.

Carmarthenshire, as a tourist destination, offers a wide range of outdoor activities. Much of the coast is fairly flat; it includes the Millennium Coastal Park, which extends for 10 miles (16 km) to the west of Llanelli; the National Wetlands Centre; a championship golf course; and the harbours of Burry Port and Pembrey. The sandy beaches at Llansteffan and Pendine are further west. Carmarthenshire has a number of medieval castles, hillforts and standing stones. The Dylan Thomas Boathouse is at Laugharne.

Stone tools found in Coygan Cave, near Laugharne indicate the presence of hominins, probably neanderthals, at least 40,000 years ago, though, as in the rest of the British Isles, continuous habitation by modern humans is not known before the end of the Younger Dryas, around 11,500 years BP. Before the Romans arrived in Britain, the land now forming the county of Carmarthenshire was part of the kingdom of the Demetae who gave their name to the county of Dyfed; it contained one of their chief settlements, Moridunum, now known as Carmarthen. The Romans established two forts in South Wales, one at Caerwent to control the southeast of the country, and one at Carmarthen to control the southwest. The fort at Carmarthen dates from around 75 AD, and there is a Roman amphitheatre nearby, so this probably makes Carmarthen the oldest continually occupied town in Wales.

Carmarthenshire has its early roots in the region formerly known as Ystrad Tywi ('Vale of [the river] Tywi') and part of the Kingdom of Deheubarth during the High Middle Ages, with the court at Dinefwr. After the Normans had subjugated England they tried to subdue Wales. Carmarthenshire was disputed between the Normans and the Welsh lords and many of the castles built around this time, first of wood and then stone, changed hands several times. Following the Conquest of Wales by Edward I, the region was reorganized by the Statute of Rhuddlan in 1284 into Carmarthenshire. Edward I made Carmarthen the capital of this new county, establishing his courts of chancery and his exchequer there, and holding the Court of Great Sessions in Wales in the town.

The Normans transformed Carmarthen into an international trading port, the only staple port in Wales. Merchants imported food and French wines and exported wool, pelts, leather, lead and tin. In the late medieval period the county's fortunes varied, as good and bad harvests occurred, increased taxes were levied by England, there were episodes of plague, and recruitment for wars removed the young men. Carmarthen was particularly susceptible to plague as it was brought in by flea-infested rats on board ships from southern France.

In 1405, Owain Glyndŵr captured Carmarthen Castle and several other strongholds in the neighbourhood. However, when his support dwindled, the principal men of the county returned their allegiance to King Henry V. During the English Civil War, Parliamentary forces under Colonel Roland Laugharne besieged and captured Carmarthen Castle but later abandoned the cause, and joined the Royalists. In 1648, Carmarthen Castle was recaptured by the Parliamentarians, and Oliver Cromwell ordered it to be slighted.