Montlhéry (French pronunciation: [mɔ̃leʁi] ⓘ) is a commune in the Essonne department in Île-de-France in northern France.

Another daughter, Alice, married into the Le Puiset family, and a son, Guy, became count of Rochefort.

In 1118, after many disputes with the rebellious lords, Louis VI of France ordered the castle to be dismantled and turned the town into a royal residence.

In the early 13th century, the French king Philip II (Augustus) completely rebuilt the castle in the contemporary style, at a new site high above the town.

During the Hundred Years' War, the town and the castle frequently passed between English and French forces.

In 1822, François Arago calculated the speed of sound there; a cannonball shot at the tower was observed from the Observatory of Villejuif.