The Luxembourg Palace (French: Palais du Luxembourg, pronounced [palɛ dy lyksɑ̃buʁ]) is at 15 Rue de Vaugirard in the 6th arrondissement of Paris, France. It was originally built (1615–1645) to the designs of the French architect Salomon de Brosse to be the royal residence of the regent Marie de' Medici, mother of King Louis XIII. After the Revolution it was refashioned (1799–1805) by Jean-François Chalgrin into a legislative building and subsequently greatly enlarged and remodeled (1835–1856) by Alphonse de Gisors. The palace has been the seat of the upper houses of the various French national legislatures (excepting only the unicameral National Assembly of the Second Republic) since the establishment of the Sénat conservateur during the Consulate; as such, it has been home to the Senate of the Fifth Republic since its establishment in 1958.

Immediately west of the palace on the Rue de Vaugirard is the Petit Luxembourg, now the residence of the Senate President; and slightly further west, the Musée du Luxembourg, in the former orangery. On the south side of the palace, the formal Luxembourg Garden presents a 25-hectare (62-acre) green parterre of gravel and lawn populated with statues and large basins of water where children sail model boats.

After the death of Henry IV in 1610, his widow, Marie de' Medici, became regent to her son, Louis XIII. Having acceded to a much more powerful position, she decided to erect a new palace for herself, adjacent to an old hôtel particulier owned by François de Luxembourg, Duc de Piney, which is now called the Petit Luxembourg and is the residence of the president of the French Senate.

Marie de' Medici desired to make a building similar to her native Florence's Palazzo Pitti; to this effect she had the architect Métezeau (either Louis Métezeau or his brother, Clément Métezeau) sent to Florence to make detailed drawings of the building.

She bought the Hôtel de Luxembourg and its fairly extensive domain in 1612 and commissioned the new building, which she referred to as her Palais Médicis, in 1615. Its construction and furnishing formed her major artistic project, though nothing remains today of the interiors as they were created for her, save some architectural fragments reassembled in the Salle du Livre d'Or. The suites of paintings she commissioned, in the subjects of which she expressed her requirements through her agents and advisers, are scattered among museums.

Marie installed her household in 1625, while work on interiors continued. The apartments in the right wing on the western side were reserved for the Queen and the matching suite to the east, for her son, Louis XIII, when he was visiting (floor plan). The 24 Marie de' Medici cycle canvases, a series commissioned from Peter Paul Rubens, were installed in the Galerie de Rubens on the main floor of the western wing. These paintings were executed between 1622 and 1625 and depicts Marie's struggles and triumphs in life. They are now visible in the Galerie Medicis of the Louvre, one of the treasures of the museum's Flemish paintings department. A series of paintings executed for her Cabinet doré ("gilded study") was identified by Anthony Blunt in 1967.

The gallery in the east wing had been intended for the display of paintings celebrating Henry IV and buildings housing stables and services were planned to either side of the pavilions flanking the entrance on the street, but these projects remained unfinished in 1631, when the Queen Mother was forced from court, following the "Day of the Dupes" in November. Louis XIII commissioned further decorations for the Palace from Nicolas Poussin and Philippe de Champaigne.

In 1642, Marie de' Medici bequeathed the Luxembourg to her second and favourite son, Gaston, Duke of Orléans, who called it the Orléans Palace (Palais d'Orléans) but by popular will it was still known by its original name. Upon Gaston's death, the palace passed to his widow, Marguerite of Lorraine, then to his elder daughter by his first marriage, Anne, Duchess of Montpensier La Grande Mademoiselle. In 1660, Anne de Montpensier sold the Luxembourg to her younger half-sister, Élisabeth Marguerite of Orléans, duchess of Guise who, in turn, gave it to her cousin, King Louis XIV, in 1694.