The Pontcysyllte Aqueduct (Welsh pronunciation: [pntkst]; Welsh: Traphont Ddr Pontcysyllte) is a navigable aqueduct that carries the Llangollen Canal across the River Dee in the Vale of Llangollen in northeast Wales.

The 18-arched stone and cast iron structure is for use by narrowboats and was completed in 1805 having taken ten years to design and build. It is 12 ft (3.7 metres) wide and is the longest aqueduct in Great Britain and the highest canal aqueduct in the world. A footpath runs alongside the watercourse on one side.The aqueduct was to have been a key part of the central section of the proposed Ellesmere Canal, an industrial waterway that would have created a commercial link between the River Severn at Shrewsbury and the Port of Liverpool on the River Mersey. Although a less expensive construction course was surveyed further to the east, the westerly high-ground route across the Vale of Llangollen was preferred because it would have taken the canal through the mineral-rich coalfields of North East Wales. Only parts of the canal route were completed because the expected revenues required to complete the entire project were never generated. Most major work ceased after the completion of the Pontcysyllte Aqueduct in 1805.

The structure is a Grade I listed building and part of a UNESCO World Heritage Site.

Thomas Telford FRS, FRSE (9 August 1757 – 2 September 1834) was a Scottish civil engineer. After establishing himself as an engineer of road and canal projects in Shropshire, he designed numerous infrastructure projects in his native Scotland, as well as harbours and tunnels. Such was his reputation as a prolific designer of highways and related bridges, he was dubbed The Colossus of Roads (a pun on the Colossus of Rhodes), and, reflecting his command of all types of civil engineering in the early 19th century, he was elected as the first President of the Institution of Civil Engineers, a post he held for 14 years until his death.

The town of Telford in Shropshire was named after him.