Merionethshire (Meirionnydd in Welsh) is a traditional county of Wales. For administrative purposes it is governed by Gwynedd and the small north-eastern Edeirnon district by Denbighshire (from 1974 to 1996 by the now-defunct Clwyd). Its own county council was abolished in 1974.

It is a maritime county, bounded to the north by Caernarfonshire, to the east by Denbighshire, to the south by Montgomeryshire and Cardiganshire, and to the west by Cardigan Bay. Its total area is 1,731 km², and it is one of the more sparsely populated counties of the UK. It is also one of the strongest Welsh-speaking parts of Wales. The coastline consists alternately of cliffs and stretches of sand and the county generally is the most mountainous in Wales; a large part of the Snowdonia National Park lies within it. The greatest heights are Aran Fawddwy 905 m (2970 ft) and Cader Idris 893 m (2929 ft). The chief rivers are the Dwyryd, the Mawddach and the Dyfi. Waterfalls and small lakes are numerous, the largest being Bala Lake (4 miles long and 1 mile broad). The main towns are Bala, Barmouth, Blaenau Ffestiniog, Corwen, Dolgellau, Ffestiniog and Tywyn. The main industries are agriculture and tourism.

Places of special interest: Castell-y-Bere (grid reference SH6608); Cymer Abbey (SH7291); Ffestiniog Railway (SH6946); Harlech Castle (SH5731); Portmeirion Italianate village (SH5837); Rhug Chapel (SJ0543); Tomen-y-mur Roman Fort (SH7038).