Westport (Irish: Cathair na Mart, meaning 'the stone fort of the beeves', historically anglicised as Cahernamart) is a town in County Mayo in Ireland. It is at the south-east corner of Clew Bay, an inlet of the Atlantic Ocean on the west coast of Ireland. Westport is a tourist destination and scores highly for quality of life. It won the Irish Tidy Towns Competition three times in 2001, 2006 and 2008. In 2012 it won the Best Place to Live in Ireland competition run by The Irish Times.

Westport is designated as a heritage town, and is one of only a few planned towns in Ireland. The town centre was laid-out in the Georgian architectural style, and incorporates the Carrow Beg river into the design composition. This provides for tree lined promenades (known as The Mall) and several stone bridges.

The pilgrimage mountain of Croagh Patrick, known locally as "the Reek", lies some 10 km west of the town near the villages of Murrisk and Lecanvey. The mountain forms the backdrop to the town.

Westport originates and gets its name in the Irish language from a 16th-century castle, Cathair na Mart (meaning "the stone fort of the beeves"). The castle and surrounding settlement belonged to the powerful local seafaring Ó Máille clan, who controlled the Clew Bay area, then known as Umaill.

The original village of Cathair na Mart existed around what is now the front (east) lawn of Westport House. It had a high street, alleys down to the river and a population of around 700. A small port also existed at the mouth of the Carrowbeg river. Roads led from the village to the west (West Road), the south (Sandy Hill Road) and the east (Old Paddock Road).

Westport's origins are tied to the Browne family. The Brownes were a noble family from Sussex, England, who arrived in Mayo in the 16th century and gradually acquired land around the county, particularly in the Westport area. Their position was strengthened in the later 17th century when Colonel John Browne, a Jacobite who had fought at the 1691 Siege of Limerick, married Maude Bourke, the great-great-granddaughter of regional soveign Grace O'Malley (Irish: Gráinne Ní Mháille), thereby inheriting rights to lands that had previously belonged to the Bourke and O’Malley families. Seeking to establish a grand residence that reflected their power, the Brownes constructed Westport House in the 18th century, initially built on the site of an Ó Máille castle at Cahernamart. The earliest version of the house stood without a lake or dam, with the tide rising and falling against its walls. From the 1730s onward, the estate was rebuilt and expanded, most notably under the direction of the German architect Richard Cassels, and later Thomas Ivory and James Wyatt. The finished house was set within parkland, with landscaped gardens, terraces, and a lake, symbolically asserting the Brownes’ authority in Mayo and anchoring the planned Georgian town of Westport that grew around the estate.

John Browne, 1st Earl of Altamont intended to move the existing Cahernamart settlement to facilitate landscaping of parklands around Westport House; this intention was outlined to Richard Pococke when he visited Browne in 1752.

The first clear evidence for the deliberate development of a new town is an advertisement in Faulkner's Dublin Journal on 17 March 1767, stating "a New Town is immediately to be built near the old town of Westport...according to Plans and Elevations already prepared". The focal point was to be a "large and elegant market house" situated in an octagonal market area enclosed by 12 "large well-finished slated Houses". There were to be "three avenues for streets of thirty slated Houses" and "several very large streets for great numbers of thatched Houses and cabbins, to be built separately" at a cost of 20–40 guineas each. Workmen were to contact Peter Brown-Kelly, son of the Earl, or the architect William Leeson.