Thuin, a town of Belgium, in that part of the province of Hainaut called "entre Sambre et Meuse." Pop. (1904), 6198. It is situated on the Sambre about 9 m. S.W. of Charleroi. The old part of the town, which dates back to the 10th century, occupies a narrow promontory between the Sambre and a small stream called the Biesmelle. The ruined tower called after him is all that remains of the fortress constructed by Bishop Notger of Liege. It was successfully defended against the Normans and long afterwards against the French under Marshal de Lorges in 1654. Although the town itself retains something of its medieval appearance it is the centre of a great manufacturing and mining district, the banks of the Sambre being lined with factories and coal-yards.