Ruthin (/ˈrɪθɪn/ ^ⓘ RITH-in; Welsh: Rhuthun) is a market town and community in Denbighshire, Wales, in the south of the Vale of Clwyd. The town, castle and St Peter's Square lie on a hill, skirted by villages such as Pwllglas and Rhewl. The name comes from the Welsh rhudd (red) and din (fort), after the colour of sandstone bedrock, from which the castle was built in 1277–1284. The Old Mill, Ruthin, is nearby. Maen Huail, a registered ancient monument associated with King Arthur and with Hueil mab Caw, the brother of the historian Gildas, stands in St Peter's Square.

There is evidence of Celtic and later Roman settlements in the area. However, little is known of the history of the town before the construction of Ruthin Castle was started in 1277 by Dafydd, the brother of prince Llywelyn ap Gruffudd. However, he forfeited the castle when he rebelled against King Edward I with his brother; Edward's queen, Eleanor, was in residence in 1281. The original name was Castell Coch yng Ngwern-fôr (Red Castle in the Sea Swamps). The Marcher Lord, Reginald de Grey, Justiciar of Chester, was given the Cantref (an administrative district) of Deffrencloyt (Dyffryn Clwyd, the Welsh for Vale of Clwyd), and his family ran the area for the next 226 years. The land dispute between Reginald Grey, 3rd Baron Grey de Ruthyn, and Owain Glyndŵr triggered Glyndŵr's rebellion against King Henry IV, which began on 16 September 1400, when Glyndŵr burned Ruthin to the ground, reputedly leaving only the castle and a few other buildings standing.

Ruthin's first parish church was St Meugan's Church, which stands in a relatively isolated location at Llanrhydd, 1 mile (1.6 km) to the south-east of the town. The site of St Meugan's has apparently been in religious use since the 6th century, although the first documented reference to the building was in 1254. It is possible that the church's location indicates an earlier focus for the settlement, which was subsequently eclipsed by the town which grew up adjoining the castle.

John Grey, 2nd Baron Grey de Wilton, established St Peter's Church in 1310 on the north side of the market place in the centre of the town, now St Peter's Square. The building dominates the Ruthin skyline. It has a double nave and boasts two medieval carved roofs. These days it is known for its musical tradition. It has a large choir of children and adults and a four-manual Wadsworth-Willis organ. Behind the church can be seen the old college buildings, school and Christ's Hospital.

A Ruthin native, Sir Thomas Exmewe was Lord Mayor of the City of London in 1517–1518.

The timber-framed Old Courthouse (built in 1401) on the square, features the remains of a gibbet last used to execute a Franciscan priest, Charles Meehan, also known as Mahoney. He was shipwrecked on the Welsh coast at a time when Catholicism was equated with treason – Meehan was hanged, drawn, and quartered in 1679. He was beatified by Pope John Paul II in 1987 as one of the Eighty-five martyrs of England and Wales.

During the English Civil War, the castle survived an eleven-week siege, after which it was demolished by order of Parliament. It was rebuilt in the 19th century as a country house, which has now been turned into the Ruthin Castle Hotel. From 1826 until 1921 the castle was the home of the Cornwallis-West family, members of Victorian and Edwardian high society.

In its 18th-century heyday as a town on drovers' routes from Wales into England, Ruthin was reputed to have "a pub for every week of the year". By 2007, however, there were only eleven pubs in the town. The public records of 23 October 1891 show 31 such establishments serving a population of 3,186; most have been converted into housing or shops. The Ruthin Union Workhouse was built in 1834.