Jules Hardouin-Mansart (Paris, April 16, 1646 – Marly, France, May 11, 1708) was a French architect whose work is generally considered to be the apex of French Baroque architecture, representing the power and grandeur of Louis XIV.

Born Jules Hardouin, he studied under his renowned great-uncle François Mansart, one of the originators of the classical tradition in French architecture; Hardouin inherited Mansart's collection of plans and drawings and adopted his well-regarded name. He also learned from Liberal Bruant, architect of the royal veteran's hospital in Paris known as Les Invalides. Hardouin-Mansart served as Louis XIV's chief architect, first enlarging the royal château of Saint-Germain-en-Laye, then at Versailles from 1675. He became the superintendent of all the royal works. He designed all the extensions and rebuildings at Versailles for the King, including the north and south wings, the Royal Chapel (with Robert de Cotte , 1710), and the celebrated Hall of Mirrors decorated by Charles Le Brun his collaborator. Outside the chateau proper he built the Grand Trianon and the Orangerie, as well as subsidiary royal dwellings not far away, such as Marly (begun 1679).

Among his other best-known works in Paris are the Pont-Royal , the great domed royal chapel Eglise du Dome dedicated to Saint Louis at Les Invalides (designed in 1680), the Place des Victoires (1684–86) followed by the Place Vendôme (1690). Most of these works still set their stamp on the character of Paris and can be seen by a modern-day tourist.

His most prominent position in France put him in place to create many of the significant monuments of the period, and to set the tone for the restrained French Late Baroque architectural style, somewhat chastened by academic detailing, that was influential as far as St. Petersburg and even echoed in Constantinople. At the same time, the size of support staff in his official bureaucratic position (illustration, above, right) has often raised criticisms that he was less than directly responsible for the work that was constructed under his name, criticisms that underestimate the discipline control within a large, classically-trained studio.

Hardouin-Mansart is considered one of the most important European architects of the 17th century.

The mansard roof, actually introduced by Francois Mansart (1598-1666), is a roof with two slopes on all four sides, with the lower slope nearly vertical and the upper nearly horizontal. At the château of Dampiere-en-Yvelines (illustration, right), Jules Hardouin-Mansart was building (1675 - 1683) for the duc de Chevreuse, Colbert's son-in-law, a patron at the center of Louis XIV's court. The "mansart" roofs are traditional slates. This French Baroque chateau of manageable size lies entre cour et jardin as even Versailles did, the paved and gravel forecourt (cour d'honneur) protected behind fine wrought iron double gates, tand enclosed by the main block and its outbuildings (corps de logis), linked by balustrades, symmetrically disposed. A traditional French touch is the modest pedimented entrance flanked by boldly projecting pavilions. Behind, the central axis is extended between the former parterres, now mown hay. The park with formally shaped water was laid out by André Le Nôtre. There are sumptuous interiors. The small scale makes it easier to compare to the approximately contemporary Het Loo, for William of Orange. These really are "Mansart roofs."

Under the influence of the neo-Baroque revival of the French Second Empire (1850 - 1870), the "mansard" became a common feature in many later 19th-century buildings in Europe and North America. In congested sites in cities, a "mansard" enabled builders to keep a decently low cornice line, while incorporating a couple of extra stories within the apparent roof. Mansards may be seen on New York City's former Grand Central Hotel (1869) (see illustration)