Joseph Simon Gallieni (24 April 1849 - 27 May 1916) was a French military leader in the French colonies and later in World War I.

He was born at Saint-Beat, in the department of Haute-Garonne. He trained at the military academy in Saint-Cyr, becoming a second lieutenant in the Marines before serving in the Franco-Prussian War of 1870-71. He was promoted to lieutenant in 1873 and captain in 1878. He was later posted to Africa in the mid-1870s, taking part in explorations and various military expeditions.

After serving in Martinique, Gallieni was made governor of French Sudan, during which time he successfully quelled a rebellion by Sudanese rebels. From 1892-96 he served in French Indochina commanding the second military division of the territory, before being dispatched to Madagascar. Here he again suppressed a revolt, this time by monarchist forces. He served as governor of Madagascar until 1905; when he resigned the governorship, Madagascar enjoyed peace and a considerable measure of prosperity, thereby earning himself a reputation as a fair, just governor.

A favored choice for supreme commander of the French Army in 1911, Gallieni declined the position in favour of Joseph Joffre, pleading advancing age and ill-health.

Retiring from the army in April 1914, Gallieni was recalled in August to assist in the defence of Paris prior to the First Battle of the Marne. Joffre, wary of Gallieni's influence and reputation, marginalised Gallieni's role to an extent. He kept him at arm's length from headquarters, although it is widely believed that Gallieni's energy and foresight was what saved Paris from the Germans.

Gallieni saw an opportunity to attack when the German First Army turned east in early September, sending the Sixth Army to strike its flank, and subsequently rushing reserves to the front by commandeered taxis in response to German counter-attacks. This was a significant factor in the subsequent French victory at the Marne. Credit for the success of the defence of Paris was, however, largely assigned to Joffre.

He subsequently served as minister of war in October 1915 before retiring, again citing ill-health in March 1916; his relationship with Joffre had proved a quarrelsome one, particularly over the tactics used at Verdun. The strain of high office having broken his already fragile health, Joseph Gallieni died in May 1916. He was posthumously appointed a Marshal of France, in 1921.