Hennef (Sieg) (German pronunciation: [ˈhɛnɛf] ) is a town in the Rhein-Sieg district of North Rhine-Westphalia, Germany. It is situated on the river Sieg, approx. 7 kilometres (4 miles) south-east of Siegburg and 15 km (9 mi) east of Bonn. Hennef is the fourth-biggest town in the Rhein-Sieg-Kreis (i.e. district). It is the site of the 15th-century castle, Schloss Allner, next to the Allner See. Within Hennef is the town of Stadt Blankenberg, with the castle of Blankenberg.