Tienen or Thienen is a city and municipality in the province of Flemish Brabant, in Flanders, one of the three regions of Belgium. The municipality comprises the city of Tienen proper and the towns of Bost, Goetsenhoven, Hakendover, Kumtich, Oorbeek, Oplinter, Sint-Margriete-Houtem and Vissenaken. On January 1, 2006 Tienen had a total population of 31,835. The total area is which gives a population density of 444 inhabitants per km².
Tienen is known in Belgium as the center of sugar production. The huge sugar beet processing factory, the Sugar refinery of Tienen (Tiense Suikerraffinaderij - Raffinerie Tirlemontoise), is located at the eastern edge of the town. The company Citrique Belge is one of the biggest producers of citric acid. Tienen has a facility for making energy saving lamps by Havells-Sylvania.
Tienen is also well known for its summer rock festival Suikerrock.
Other cultural landmarks include the swimming pool and local community centre.