Cardiganshire (Sir Aberteifi in Welsh) is a traditional county in Wales that came into being in 1282.

In 1282 Edward I conquered the principality of Wales and divided the area into counties. The name Cardigan was an anglicization of the name for the historic kingdom of Ceredigion.

Cardiganshire's county council was abolished in 1974 by the Local Government Act 1972 — and it was succeeded by the district of Ceredigion in the administrative county of Dyfed. This district was split out in 1996 as a unitary authority, has (bar minor realignments) identical borders to traditional Cardiganshire, and is sometimes known by that name.