Ceredigion (Welsh: [kɛrɛˈdɪɡjɔn] ^ⓘ), historically Cardiganshire (/ˈkɑːrdɪɡənˌʃɪər, -ʃər/, Welsh: Sir Aberteifi), is a county in the west of Wales. It borders Gwynedd across the Dyfi estuary to the north, Powys to the east, Carmarthenshire and Pembrokeshire to the south, and the Irish Sea to the west. Aberystwyth is the largest settlement and, together with Aberaeron, is an administrative centre of Ceredigion County Council.

The county is the second most sparsely populated in Wales, with an area of 688 square miles (1,780 km²) and a population of 71,500; the latter is a decline of 4,492 since the 2011 census. After Aberystwyth (15,935), the largest towns are Cardigan (4,184) and Lampeter (2,970). Ceredigion is considered a centre of Welsh culture, and as of the 2021 census, 45.3% of the population could speak the Welsh language.

To the west, Ceredigion has 50 miles (80 km) of coastline on Cardigan Bay, which is traversed by the Ceredigion Coast Path. Its hinterland is hilly and rises to the Cambrian Mountains in the east, where the highest point is Plynlimon at 752 metres (2,467 ft). The mountains are the source of the county's main rivers: the Rheidol, Ystwyth, Aeron and Teifi; the last of these is Ceredigion's boundary with Carmarthenshire and Pembrokeshire for most of its length.

Ceredigion is named after a minor kingdom which occupied approximately the area of the county in the fifth century AD. The contemporary county has the same borders as Cardiganshire, which was established in 1282 by the English king Edward I after his conquest of Wales. In the 18th and early 19th centuries, the county was more industrialised than it is today; lead, silver and zinc were mined in the area, and Cardigan was the largest port in South Wales. The economy later became highly dependent on dairy farming, but as farming becomes less profitable, it is diversifying into areas such as tourism. The county is home to the National Library of Wales, Aberystwyth University, and the Lampeter campus of University of Wales Trinity St David.

Ceredigion has been inhabited since prehistoric times. A total of 170 hill forts and enclosures have been identified across the county and there are many standing stones dating back to the Bronze Age. Around the time of the Roman invasion of Britain, the area was between the realms of the Demetae and Ordovices. The Sarn Helen road ran through the territory, with forts at Bremia and Loventium protecting gold mines near present-day Llanddewi Brefi. Following the Roman withdrawal, Irish raids and invasions were repulsed, supposedly by the forces under a northerner named Cunedda. The 9th-century History of the Britons attributed to Nennius records that Cunedda's son Ceredig settled the area around the Teifi in the 5th century. The territory supposedly remained a minor kingdom under his dynasty until its extinction upon the drowning of Gwgon ap Meurig c. 871, after which it was administered by Rhodri Mawr of Gwynedd before passing to his son Cadell, whose son Hywel Dda inherited its neighbouring kingdom Dyfed and established the realm of Deheubarth. Records are highly obscure; some historians believe that Hyfaidd ap Bledrig, the Dyfed ruler, may have annexed Ceredigion before his heirs lost it to Hywel through war.

Many pilgrims passed through Cardiganshire on their way to St Davids. Some came by sea and made use of the churches at Mwnt and Penbryn, while others came by land seeking hospitality at such places as Strata Florida Abbey. Both the abbey and Llanbadarn Fawr were important monastic sites of scholarship and education. Place names including ysbyty denote their association with pilgrims.

In 1282, Edward I of England conquered the principality of Wales and divided the area into counties. One of thirteen traditional counties in Wales. Cardiganshire was split into the five hundreds of Genau'r-Glyn, Ilar, Moyddyn, Penarth and Troedyraur.

Much later, Cardiganshire was designated as a vice-county.