The French Third Republic was the system of government adopted in France from 4 September 1870, when the Second French Empire collapsed during the Franco-Prussian War, until 10 July 1940, after the Fall of France during World War II led to the formation of the Vichy government.

The early days of the Third Republic were dominated by political disruptions caused by the Franco-Prussian War of 1870–1871, which the Republic continued to wage after the fall of Emperor Napoleon III in 1870. Harsh reparations exacted by the Prussians after the war resulted in the loss of the French regions of Alsace (keeping the Territoire de Belfort) and Lorraine (the northeastern part, i.e. present-day department of Moselle), social upheaval, and the establishment of the Paris Commune. The early governments of the Third Republic considered re-establishing the monarchy, but disagreement as to the nature of that monarchy and the rightful occupant of the throne could not be resolved. Consequently, the Third Republic, originally envisioned as a provisional government, instead became the permanent form of government of France.

The French Constitutional Laws of 1875 defined the composition of the Third Republic. It consisted of a Chamber of Deputies and a Senate to form the legislative branch of government and a president to serve as head of state. Calls for the re-establishment of the monarchy dominated the tenures of the first two presidents, Adolphe Thiers and Patrice de MacMahon, but growing support for the republican form of government among the French populace and a series of republican presidents in the 1880s gradually quashed prospects of a monarchical restoration.

The Third Republic established many French colonial possessions, including French Indochina, French Madagascar, French Polynesia, and large territories in West Africa during the Scramble for Africa, all of them acquired during the last two decades of the 19th century. The early years of the 20th century were dominated by the Democratic Republican Alliance, which was originally conceived as a centre-left political alliance, but over time became the main centre-right party. The period from the start of World War I to the late 1930s featured sharply polarized politics, between the Democratic Republican Alliance and the Radicals. The government fell less than a year after the outbreak of World War II, when Nazi forces occupied much of France, and was replaced by the rival governments of Charles de Gaulle's Free France (La France libre) and Philippe Pétain's French State.

During the 19th and 20th centuries, the French colonial empire was the second largest colonial empire in the world only behind the British Empire.