Monmouthshire is one of thirteen historic counties of Wales, covering south-east Wales. It was formed from the Welsh Marches by the Laws in Wales Act 1535.

The county borders Gloucestershire to the east, Herefordshire to the northeast, Brecknockshire to the north, and Glamorgan to the west. The parish of Welsh Bicknor, situated a short distance east of Monmouthshire's eastern border, sandwiched between the borders of Gloucestershire and Herefordshire, was considered part of Monmouthshire until it was made part of Herefordshire "for all purposes" by the Counties (Detached Parts) Act 1844.

Under the Local Government Act 1972, the use of Monmouthshire for local government and ceremonial purposes ended on April 1, 1974, although it remains in use as a general geographic area and for other purposes, such as a vice county for biological recording.

A local government principal area named Monmouthshire was created on April 1, 1996, covering the eastern 60% of the historic county.

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. The current preserved county of Gwent is similar in extent to the traditional county of Monmouthshire with the addition of the Rhymney Valley area.