Thuin (French pronunciation: [tɥɛ̃] ⓘ or [twɛ̃]; Walloon: Twin) is a city and municipality of Wallonia located in the province of Hainaut, Belgium.

The municipality consists of the following districts: Biercée, Biesme-sous-Thuin, Donstiennes, Gozée, Leers-et-Fosteau, Ragnies, Thuillies, and Thuin (including the hamlets of Hourpes and Maladrie.

The earliest name of the settlement, Thudinium Castellum, referring to a Roman fortification, is found on a 9th-century offering in Lobbes Abbey, which lists various neighbouring towns and related tithe duties.

The neighbouring Aulne Abbey, reputedly founded in the 7th century by Landelin, a repentant robber, was also made part of the Bishopric of Liège.

A century later, Prince-Bishop Notger had a defensive wall built in Thuin, which then became the westernmost of the 23 bonnes villes (or principal cities) of the bishopric.