Monmouthshire (Welsh: Sir Fynwy) is a county in south east Wales. The name derives from the historic county of Monmouthshire of which it covers the eastern 60%. The largest town is Abergavenny. Other towns and large villages are Caldicot, Chepstow, Monmouth, Magor and Usk.

The historic county of Monmouthshire was formed from the Welsh Marches by the Laws in Wales Act 1535 and bordered Gloucestershire to the east, Herefordshire to the northeast, Brecknockshire to the north, and Glamorgan to the west. The Laws in Wales Act 1542 again enumerated the counties of Wales and omitted Monmouthshire, implying that the county was no longer to be treated as part of Wales. However, for all purposes Wales had become part of the Kingdom of England, and the difference had little practical effect. For several centuries, acts of the Parliament of England (in which Wales was represented) often referred to "Wales and Monmouthshire". However, the Local Government Act 1972, which came into effect in April 1974, confirmed the county as part of Wales, with the administrative county of Monmouthshire and its associated lieutenancy being abolished. Most of its area was transferred to a new local government and ceremonial county called Gwent, with the same eastern and southern boundaries as the historic county, the River Wye and the Severn Estuary, while the western two-fifths of the former Monmouthshire are now administered by other Welsh unitary authorities: Blaenau Gwent, Torfaen, Caerphilly, and Newport.

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