The Gudenå or Gudenåen (pronounced [ˈkuˀðn̩ˌɔˀn̩]), is Denmark's longest river and runs through the central parts of the Jutlandic peninsula. An anglicized version of the name often seen is 'The River Guden'.

The Gudenåen has its spring in Tinnet Krat, Vejle Municipality (between Nørre Snede and Tørring-Uldum) and flows a total of 149 kilometres (93 mi) to Randers Fjord in Randers, on a northward course which takes it through the central parts of Jutland. On its way, the river traverses the relatively high lying region of Søhøjlandet, through the lakes of Naldal Sø, Vestbirk Sø, Mossø, Gudensø, Rye Mølle Sø, Birksø, Julsø, Borre Sø, Brassø, Silkeborg Langsø and Sminge Sø before it empties in Randers Fjord; a long inlet of the Kattegat sea. It is fed by numerous streams and wetlands along the way.

The Gudenåen came into existence some 15,000 years ago, at the end of the last Ice Age, when melting ice and glacial streams carved out its bed. The river shelters many species of animals and parts of its course are to be protected under the regulations of Natura 2000.

• The Gudenaa Committee
• Virtual journey on Gudenåen in Google Earth
• Gudenåkomitéen A map of the river. (in Danish)

56°29′N 10°13′E / 56.483°N 10.217°E / 56.483; 10.217