The River Spey (Gaelic: Uisge Spè) is the second longest and the fastest flowing river in Scotland. It is important for salmon fishing and whisky production.

Rising at over 1000 feet (300 m) at Loch Spey in Corrieyairack Forest in the Scottish Highlands, 10 miles (16 km) south of Fort Augustus, it descends to flow north-east to the Moray Firth.

The River Spey is unusual in that it increases speed as it flows closer to the coast, due to the surrounding geography. The mean flow is around 16 m/s making it the fastest flowing river in Scotland, possibly the UK (depending on what constitutes a river.) The Spey does not meander, although it does rapidly move its banks, either (relatively) gradually as a result of deposition and erosion from normal flow, or in a matter of hours as a result of going into spate. The Muckle (or Great) Spate of 1829 devastated much of Strathspey and Speyside including many of the bridges on the river. The Spey spates quickly due to its wide catchment area in the mountains, as a result of rainfall or snowmelt. South of Fochabers the banks are reinforced with a high earth barrier, but this has been broken by the river on several occasions, removing a large portion of Garmouth Golf Course, sections of wall surrounding Gordon Castle, parts of the Speyside Way and some of the B9104 road. The Spey railway bridge (now pedestrianised) Spey Bay to Garmouth was originally designed with the main span over the main flow of the river, however before construction was completed the river had changed its course and was running at one end of the bridge.

Insh Marshes, areas of roughly two miles on either side of the Spey in the lower reaches are designated a Site of Special Scientific Interest, as are the extensive shingle systems at Spey Bay.

The river is also known for the quality of its salmon and trout fishing, including a particular genre of fly fishing that uses a unique two-handed fly rod (commonly known as a 'spey rod') and the 'spey casting' style, both developed in Scotland.

Speyside distilleries produce more whisky than any other region.

The Speyside Way is a long-distance footpath that follows the River Spey through some of Morayshire's most beautiful scenery.

Ptolemy named the river as Tuesis Aest.