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Isleworth is a small town of Saxon origin sited within the London Borough of Hounslow in west London, England. It lies immediately east of the town of Hounslow and west of the River Thames and its tributary the River Crane. Isleworth's original area of settlement, alongside the Thames, is known as 'Old Isleworth'. The north-west corner of the town, bordering on Osterley to the north and Lampton to the west, is known as 'Spring Grove'.

Isleworth's former Thames frontage of approximately one mile, excluding that of the Syon estate, was reduced to little over half a mile in 1994 when a borough boundary realignment was effected in order to unite the district of St Margarets wholly within London Borough of Richmond upon Thames. As a result, most of Isleworth's Thames-side is that part overlooking the 3.5-hectare (8.6-acre) islet of Isleworth Ait: the short-length River Crane flows into the Thames south of the Ait, and its artificial distributary the Duke of Northumberland's River west of the Ait, one of two Colne distributaries constructed for aesthetic reasons in the 1600-1750 period.

The pronunciation of the first syllable of the place has no relation to any 'isle', see the person's name below from which it derives, and is an instance of a counterintuitive place name within Greater London, alongside Greenwich, Southwark, Marylebone, Plaistow, and often Holborn; however in this instance it can be argued that the similar word is counterintuitively pronounced, & the place name is literal.