Monmouthshire (Welsh: Sir Fynwy) is both a principal area and a traditional county in south-east Wales.

The principal area
The current unitary authority was created in 1996 and covers the eastern half of the traditional county, including the following towns:

• Abergavenny
• Caldicot
• Chepstow
• Monmouth
• Usk

It was formed on April 1, 1996 as a successor to the previous district of Monmouth along with a small part of the former Blaenau Gwent district in the administrative county of Gwent, which themselves were created in 1974.

The council's administrative headquarters are in Cwmbran — outside of its own jurisdiction in the neighbouring borough of Torfaen.

The traditional county
The traditional county of Monmouthshire includes Newport, and borders Gloucestershire to the east, Herefordshire to the northeast, Brecknockshire to the north, and Glamorgan to the west. The county also includes the exclave of Welsh Bicknor, situated a short distance east of Monmouthshire's eastern border, sandwiched between the borders of Gloucestershire and Herefordshire. For administrative purposes this has been part of Herefordshire since 1844.

The county is traditionally divided into six hundreds:

• Abergavenny
• Caldicot
• Raglan
• Skenfrith
• Usk
• Wentloog

The chief rivers are the Wye (much of which forms the border with Gloucestershire), the Usk, and the Rhymney (which forms the border with Glamorgan). The county has a diverse industrial base including agriculture, electronics, engineering, tourism and service industries.

History
Monmouthshire's Welsh status was ambiguous until the 1960s. Previously, the legal formula had been to refer to 'Wales and Monmouthshire'. In popular usage, it had been considered part of Wales for many centuries. The ambiguity surrounding its status arose from its not being mentioned in the second Laws in Wales Act in the 16th century. The 1911 Encyclopædia Britannica unambigiously described the county as part of England, but notes that 'whenever an act [...] is intended to apply to [Wales] alone, then Wales is always coupled with Monmouthshire'.

The Acts that defined Monmouthshire did treat it in a slightly different way to other counties created out of the Marches (for example, it sent two members to the Commons, like English counties, rather than one, like the other Welsh ones). However, this is something of an irrelevance, as the entirety of Wales and the Marches had been part of England since the Statute of Rhuddlan in 1284.

The question of Monmouthshire's status continued to be a matter of discussion, especially as Welsh nationalism and devolution climbed the political agenda in the 20th century: nonetheless, in the rare event that an Act of Parliament was restricted to Wales, Monmouthshire was always included, and the creation of the Welsh Office in 1964 explicitly included Monmouthshire. A typical example was the division of England and Wales into registration areas in the 19th century - one of which, the "Welsh Division", was defined as including "Monmouthshire, South Wales and North Wales".

The question was clarified in law by an Order in Council of 1968, and further clarified by the Local Government Act 1972, which provided that in legislation after 1974 the definition of "Wales" would include it. The Interpretation Act 1978 provides that in legislation passed between 1967 and 1974, "a reference to England includes Berwick upon Tweed and Monmouthshire", but would exclude the rest of Wales.

Being a part of the diocese of Llandaff, Monmouthshire was included in the area in which the Church of England was disestablished in 1920 to become the Church in Wales.