Joseph Simon Gallieni (24 April 1849 – 27 May 1916) was a French military officer, active for most of his career as a military commander and administrator in the French colonies where he wrote several books on colonial affairs.

He was recalled from retirement at the beginning of the First World War. As military governor of Paris he played an important role in the First Battle of the Marne, when Maunoury's Sixth Army, which was under his command, attacked the German west flank. A small portion of its strength was rushed to the front in commandeered Paris taxicabs.

From October 1915 he served as Minister of War, resigning from that post in March 1916 after criticizing the performance of the French Commander-in-Chief, Joseph Joffre (formerly his subordinate, earlier in their careers), during the German attack on Verdun. He died later that year and was made Marshal of France posthumously in 1921.

Gallieni was born in 1849 at Saint-Béat, in the department of Haute-Garonne, in the central Pyrenees. He was of Corsican and Italian descent. His father, born in Pogliano Milanese, had risen from the ranks to be a captain.

As a student, he was educated, hard-working, and studious at the Prytanée Militaire in La Flèche, and then the École Spéciale Militaire de Saint-Cyr. He was also gifted and outstanding in mathematics and languages. He later became a second lieutenant in the 3rd Marine Infantry Regiment before serving in the Franco-Prussian War.

Gallieni fought at Sedan and was taken prisoner at Bazeilles, scene of the stand of the colonial marines. He learned German while a prisoner there, and later kept a notebook in German, English and Italian called "Erinnerungen of my life di ragazzo" ("Memories of my life from boyhood [onwards]").

He was promoted to lieutenant in 1873. His colonial career began in 1876 in Senegal. He was promoted to captain in 1878. He led an expedition to the upper Niger. He also served in Reunion and Martinique. In 1886, he had risen to the rank of lieutenant-colonel, and was later appointed governor of the French Sudan, during which time he successfully quelled a rebellion by Sudanese insurgents under Mahmadu Lamine. He was outstanding at colonial penetration without open hostilities in West Africa in 1880 and 1886–8.

In 1888 he was appointed to the War College. In 1892-6 he served as a colonel in French Indochina commanding the second military division of the territory in Tonkin. In 1894 he led successful French action against the nationalist leader Đề Thám, but further military action was overruled by colonial administrators after Đề Thám was accorded a local fiefdom.