Monmouthshire (/ˈmɒnməθʃər, ˈmʌn-, -ʃɪər/ MON-məth-shər, MUN-, -⁠sheer; Welsh: Sir Fynwy) is a county in the south east of Wales. It borders Powys to the north; the English counties of Herefordshire and Gloucestershire to the north and east; the Severn Estuary to the south, and Torfaen, Newport and Blaenau Gwent to the west. The largest town is Abergavenny, and the administrative centre is Usk.

The county is rural, although adjacent to the city of Newport and the urbanised South Wales Valleys. It has an area of 330 square miles (850 km²) and a population of 94,930. Abergavenny is located in the north-east, and Usk in near the centre; other settlements include Monmouth in the east, Chepstow in the south-east, Caldicot in the south. Monmouthshire County Council is the local authority. Monmouthshire was established in 1996 and is named after the historic county of the same name, of which it covers approximately the eastern three-fifths. The county has one of the lowest percentages of Welsh speakers in Wales, at 8.2% of the population in 2021.

The lowlands in the centre of Monmouthshire are gently undulating, and shaped by the River Usk and its tributaries. The west of the county is hilly, and the Black Mountains in the north-west are part of the Brecon Beacons National Park. The border with England in the east largely follows the course of the River Wye and its tributary, the River Monnow. In the south east is the Wye Valley, a hilly region which stretches into England and which has been designated an Area of Outstanding Natural Beauty. The county has a shoreline on the Severn Estuary, with crossings into England by the Severn Bridge and Prince of Wales Bridge.

The Gwent Levels in the south of the county contain signs of human occupation dating back eight millennia. During the Iron Age, south-east Wales was settled by the Silures, who opposed the Roman conquest of Britain before being subdued. The ruins of Venta Silurum, in modern-day Caerwent, are evidence of Roman rule. The sub-Roman history of the county is poorly documented but saw the founding of petty kingdoms, including Gwent. The area was part of the Welsh Marches during the Middle Ages and was frequently contested, as reflected in its "fine collection" of castles. In the seventeenth century, Raglan Castle was among the last Royalist strongholds to fall to Parliamentarian forces in the English Civil War, and the county became a recusant stronghold in which Catholics were widely persecuted. In the mid-19th century, John Frost and other Chartist leaders were tried and sentenced to death at the Shire Hall, Monmouth after the "first mass movement of the working class" in Britain. At the same time, the Wye Tour and the ruins Tintern Abbey drew tourists to the county. In the 21st century the economy is based on the service sector, agriculture and tourism.

Evidence of human activity in the Mesolithic period has been found across Monmouthshire; examples include remains on the Caldicot and Wentloog Levels and at Monmouth. A major hoard of Bronze Age axes was discovered at St Arvans. The county has a number of hillfort sites, such as those at Bulwark and Llanmelin Wood. The latter has been suggested as the capital of the Silures, a Celtic tribe who occupied south-east Wales in the Iron Age. The Silures proved among the most intractable of Rome's opponents; Tacitus described them as "exceptionally stubborn" and Raymond Howell, in his county history published in 1988, notes that while it took the Romans five years to subdue south-east England, it took thirty-five before complete subjugation of the Silurian territories was achieved.

The Roman conquest of Britain began in AD 43, and within five years the Roman Empire's legions had reached the borders of what is now Wales. In south east Wales they encountered strong resistance from the Silures, led by Caratacus (Caradog), who had fled west after the defeat of his own tribe, the Catuvellauni. His final defeat in AD 50 saw his transportation to Rome, but Silurian resistance continued, and the subjugation of the entirety of south east Wales was not achieved until around AD 75, under the governor of Britain, Sextus Julius Frontinus.

Monmouthshire's most important Roman remains are found at the town of Venta Silurum ("Market of the Silures"), present-day Caerwent in the south of the county. The town was established in AD 75, laid out in the traditional rectangular Roman pattern of twenty insulae with a basilica and a temple flanking a forum. Other Roman settlements in the area included Blestium (Monmouth). The Romanisation of Monmouthshire was not without continuing civil unrest; the defences at Caerwent, and at Caerleon, underwent considerable strengthening in the late 2nd century in response to disturbances. The Silurian identity was not extinguished: the establishment of a Respublica Civitatis Silurium (an early town council) at the beginning of the 3rd century testifies to the longevity of the indigenous tribal culture.

The Roman abandonment of Britain from AD 383 saw the division of Wales into a number of petty kingdoms. In the south east (the present county of Monmouthshire) the Kingdom of Gwent was established, traditionally by Caradoc, in the 5th or 6th centuries. Its capital, Caerwent, gave the name to the kingdom. The subsequent history of the area prior to the Norman Conquest is poorly documented and complex. The Kingdom of Gwent frequently fought with the neighbouring Welsh kingdoms, and sometimes joined in alliance with them in, generally successful, attempts to repel the Anglo-Saxons, their common enemy. The Book of Llandaff records such a victory over the Saxon invaders achieved by Tewdrig at a battle near Tintern in the late 6th century. An example of the alliances formed by neighbouring petty kings was the Kingdom of Morgannwg, a union between Gwent and its western neighbour, the Kingdom of Glywysing, which formed and reformed between the 8th and the 10th centuries. The common threat they faced is shown in Offa's Dyke, the physical delineation of a border with Wales created by the Mercian king. For a brief period in the 11th century, Monmouthshire, as Gwent, became part of a united Wales under Gruffydd ap Llywelyn, but his death in 1063 was soon followed by that of his opponent Harold Godwinson at the Battle of Hastings, and the re-established unity of the country was to come from Norman dominance.