Llanfyllin ( Welsh pronunciation (help·info)) is a small town in Powys, Mid Wales, United Kingdom. Llanfyllin's population at the date of the 2001 Census was 1,407, of which over half have knowledge of the Welsh language.

The town lies in a valley by the Berwyn Mountains in Montgomeryshire, 14 miles (23 km) southwest of Oswestry, 15 miles (24 km) from Montgomery, and 180 miles (290 km) from London. The valley is meanered by two streams, named the rivers Cain and Abel, tributaries which join the River Vyrnwy at Llansantffraid.

The town sits on the main route between Shrewsbury and Bala, Gwynedd, long time key markets towns in this trading area of Wales and the Welsh borders. In the suburb of Bodyddon, there exists evidence of both an early Britons settlement, and a Roman road which leads directly towards Offas Dyke.

The town is known for its holy well, dedicated to Saint Myllin who baptised people at Fynnon Coed y Llanin, which drew early settlers.

Granted its charter by Llywelyn ap Dafydd under Edward I, it was confirmed by Edward de Charlton, Lord of Powys under Henry V, making Llanfyllin a market town. In 1644, Charles I spent a night in Llanfyllin, and after having lunch the following day at Brithdir, continued his journey to Chirk Castle.