Llanwrtyd Wells (Welsh: Llanwrtyd) is a small town in Powys, in the traditional county of Brecknockshire, mid Wales, lying on the River Irfon . With a population of about 700 people, it claims to be the smallest town in Britain, although Fordwich in Kent has a smaller population.

Llanwrtyd Wells grew as a spa town, based around the Ffynon Droellwyd spring, which still exists. The town was also known as an eisteddfod site, and is the site of a bog snorkelling competition and the annual Man versus Horse Marathon, as well as many other annual events.