Merionethshire ( Welsh: Sir Feirionnydd ) is one of the thirteen traditional counties and a former administrative county of Wales. Historical capital is the town of Dolgellau.
The county is located on the Cardigan Bay and is bordered on the land side of Caernarfonshire, Denbighshire, Montgomeryshire and Cardiganshire. The landscape is mountainous and is dominated by the Snowdonia National Park. The highest elevations are the Aran Fawddwy (905 m) and the Cadair Idris ( 893 m). The Dwyryd, the Mawddach and Dyfi which are the main rivers. Significant places besides Dolgellau are Aberdovey, Bala, Barmouth, Blaenau Ffestiniog, Corwen, Ffestiniog, Harlech and Tywyn.
1961 the county had 39 007 inhabitants in 1709 km ².
By 1974, Merionethshire was an administrative county and was then divided on the Meirionnydd district of Gwynedd and Clwyd District of Glyndŵr. Since the administrative reform of 1996, the area of Merionethshire over the unitary authorities of Gwynedd and Denbighshire extends.