Montreuil or Montreuil-sur-Mer (French pronunciation: ​[mɔ̃tʁœj syʁ mɛʁ]; Dutch: Monsterhole) is a sub-prefecture in the Pas-de-Calais department in northern France. It is located on the Canche river, not far from Étaples. The sea, however, is now some distance away.

Montreuil is surrounded by notable brickwork ramparts, constructed after the destruction of the town by troops of Habsburg emperor Charles V in June 1537.^[2] These fortifications pre-date the extensive fortification of towns in northern France by Sébastien Le Prestre de Vauban in the 17th century.

Population (1999): 2,688 inhabitants for the city, 21,603 inhabitants for the canton and 99,288 inhabitants for the arrondissement.

Montreuil was the headquarters of the British Army in France during the First World War from March 1916 until it closed in April 1919. The military academy there provided excellent facilities for GHQ.

Montreuil was chosen as G.H.Q. for a wide variety of reasons. It was on a main road from London to Paris—the two chief centers of the campaign—though not on a main railway line, which would have been an inconvenience. It was not an industrial town and so avoided the complications alike of noise and of a possibly troublesome civil population. It was from a telephone and motor transit point of view in a very central situation to serve the needs of a Force which was based on Dunkirk, Calais, Boulogne, Dieppe, and Havre, and had its front stretching from the Somme to beyond the Belgian frontier.^[3]

Haig staff member Sir Frank Fox OBE wrote a critically acclaimed contemporary account of the headquarters in 1916, originally published under the pseudonym "GSO", called G.H.Q. (Montreuil-sur-Mer) His work in the QMG's Directorate in the final offensive against the German Army resulted in his being awarded the OBE (Military) He was also Mentioned in Despatches.

General Haig was quartered in the nearby Château de Beaurepaire, two miles (3.2 kilometres) SE of the town on the D138. There is a plaque on the château wall to commemorate the event.

King George V, accompanied by Haig, made a triumphant passage through Montreuil on his way to Paris on 27 November 1918.^[4]

A statue of Haig on horseback, commemorating his stay, can be seen outside the theatre on the Place Charles de Gaulle.^[5] During the German occupation of the town during the Second World War, the statue was taken down. It was never found and is thought to have been melted down. It was rebuilt in the 1950s, using the sculptor's original mould.

Lawrence Sterne visited the town in 1765. He recounted his visit through the eyes of the narrator of his novel A Sentimental Journey Through France and Italy (1768).

Montreuil is the setting for part of Victor Hugo's novel Les Misérables, where it is identified only as M____-sur-M__ in past translations. The protagonist, Jean Valjean (going by the name Monsieur Madeleine), is for a few years the mayor of Montreuil, as well as owner of the local factory, and it is where the character Fantine lives, works, and later becomes a prostitute before dying in a local hospital. Hugo had spent several vacations in Montreuil.^[6]

Montreuil is twinned with: