Monmouth Castle (Welsh: Castell Trefynwy) is a castle close to the centre of the town of Monmouth, the county town of Monmouthshire, on a hill above the River Monnow in south east Wales.

Once an important border castle, and birthplace of Henry V of England, it stood until the English Civil War when it was damaged and changed hands three times before being slighted to prevent it being fortified again. After partial collapse in 1647, the site was reused and built over by Great Castle House, which became the headquarters and regimental museum of the Royal Monmouthshire Royal Engineers.

It is a Grade I listed building and scheduled monument.

Great Castle House was built in 1673, on the site of the old round tower, by Henry Somerset, 1st Duke of Beaufort. It is a Grade I listed building, and has been described as "a house of splendid swagger outside and in". It later became used for the Assize Courts, until they relocated to the new Shire Hall in 1725.
Only fragments of the castle, including the Great Tower and Hall and parts of the walls, remain above ground, and on the site Castle House and Great House have been built. In 1875, the Royal Monmouthshire Royal Engineers Militia, the senior Territorial Army regiment today, made it their Headquarters building and so it remains.

It is one of the few British castles in continuous military occupancy. The Royal Monmouthshire Royal Engineers museum is located in the stable block attached to Great Castle House. It includes exhibits relating to the history of the regiment from 1539 to the present day.
The castle is a Grade I listed building[26] and a scheduled monument in the care of Cadw.