The Luxulyan Valley is the steep sided and thickly wooded valley of the River Par, situated in the south east of Cornwall, England, United Kingdom. It contains a major concentration of early 19th century industrial remains, and was designated as part of a World Heritage Site in 2006. The valley stretches to the south-east of the village of Luxulyan, reaching as far as the edge of the built-up area around the town of St Blazey and the port of Par, a distance of some 5 km.

Most of the industrial remains in the valley are the results of the endeavours of Joseph Treffry (1782-1850). Copper mining was booming in the area during the early 1800s, and Treffry was the owner of Fowey Consols mine, one of the deepest, richest and most important of the Cornish copper mines. The mine was situated to the east of the southern end of the Luxulyan Valley proper, and its site is part of the designated World Heritage Site. At its peak it was worked by six steam engines and 17 water wheels.

In the late 1820s, Treffry built the port of Par Harbour. He then linked this to Pontsmill, at the southern end of the valley, by means of the Par Canal. A mineral tramway connected the canal head to the mine. A second tramway followed in 1835, via an inclined plane, a flat route along the eastern lip of the valley, and the major Treffry Viaduct across the valley to Luxulyan. This second route served two major purposes, as it enabled Treffry to develop Carbeans and Colcerrow granite quarries in the upper valley, and was also used by a leat carrying water to supply the mine at Fowey Consols. Two further granite quarries, known as Rock Mill and Orchard, operated lower down in the valley. In 1870 these were linked to Pontsmill by a third tramway along the valley floor.

The 1835 tramway eventually became part of the Cornwall Minerals Railway which linked the English Channel ports of Par and Fowey with the china clay workings of central Cornwall, and to the Atlantic port of Newquay. As part of this process, the section of the route between Pontsmill and Luxulyan, with its incline and flat sections ideal for animal haulage, was replaced with a more gradually climbing route through the valley itself, more suited to locomotive haulage. The newer route is still in use, as part of the railway from St. Blazey to Newquay, and passes beneath the spans of the Treffry Viaduct that carried its predecessor. The older tramway routes remained in use to serve the various granite quarries until the early 20th century.

The Treffry Viaduct of 1847 is a granite structure 216 yds. long and 98 ft. high. It has ten 40 ft. span semi-circular arches and also carried a stone water channel between the rails. This was to power a water wheel.

The viaduct was in occasional use until 1933. It is now a scheduled Ancient Monument.