Carlow (/ˈkɑːrloʊ/ KAR-loh; Irish: Ceatharlach [ˈcahəɾˠl̪ˠəx]) is the county town of County Carlow, in the south-east of Ireland, 84 km (52 mi) from Dublin. At the 2022 census, it had a population of 27,351, the twelfth-largest urban center in Ireland.

The River Barrow flows through the town and forms the historic boundary between counties Laois and Carlow. However, the Local Government (Ireland) Act 1898 included the town entirely in County Carlow. The settlement of Carlow is thousands of years old and pre-dates written Irish history. The town has played a major role in Irish history, serving as the capital of the country in the 14th century. The town is in a townland and civil parish of the same name.

The name is an anglicisation of the Irish Ceatharlach. Historically, it was anglicised as Caherlagh, Caterlagh and Catherlagh, which are closer to the Irish spelling. According to logainm.ie, the first part of the name derives from the Old Irish word cethrae ("animals, cattle, herds, flocks"), which is related to ceathar ("four") and therefore signified "four-legged". The second part of the name is the ending -lach, that makes abstract collective nouns.

Some believe that the name should be Ceatharloch (meaning "quadruple lake"), since ceathar means "four" and loch means "lake". It is directly translated as "Four lakes", although, there is seemingly no evidence to suggest that these lakes ever existed in this area.

Evidence shows that human occupation in the Carlow county area extends back thousands of years. The most notable and dramatic prehistoric site is the Browneshill Dolmen – a megalithic portal tomb just outside Carlow town.

Now part of the diocese of Kildare and Leighlin, several Early Christian settlements are still in evidence today around the county. St Mullin's monastery is believed to have been established around the 7th century, the ruins of which are still in evidence today. Old Leighlin was the site of one of the largest monastic settlements in Ireland and the location for a church synod in 630 AD which determined the date of Easter. St Comhgall built a monastery in the Carlow area in the 6th century, an old church building and burial ground survive today at Castle Hill known as Mary's Abbey. Carlow was an Irish stronghold for agriculture in the early 1800s which earned the county the nickname of the scallion eaters. Famine later wiped out half of the population.

Carlow Castle was constructed by William Marshal, Earl of Striguil and Lord of Leinster, c. 1207 – c. 1213, to guard the vital river crossing. It was also to serve as the capital of the Lordship of Ireland from 1361 until 1374. This imposing structure survived largely intact until 1814 when it was mostly destroyed in an attempt to turn the building into a lunatic asylum. The present remains now are the West Wall with two of its cylindrical towers.

Carlow was incorporated as a borough in 1296 by Edward I. The parliamentary borough returned two MPs to the Irish House of Commons until 1801 and its successor constituency returned one MP to the United Kingdom House of Commons from 1801 to 1885.