Adolphe Niel (October 4, 1802 - August 13, 1869) was a marshal of France.

He was born at Muret , and entered the École Polytechnique in 1821, passing on to the engineer school at Metz, becoming lieutenant in the Engineers in 1827 and captain in 1833. At the storming of Constantine he led the engineer detachment with one of the storming parties, and his conduct gained for him the rank of chef de bataillon (1837). In 1840 he was promoted lieutenant-colonel, and in 1846 colonel, and his next war service was as chief of staff to General Vaillant during the siege of Rome (1849), after which he was made general of brigade and director of engineer services at headquarters.

In 1851 he became a member of the Committee of Fortifications , in the following year a member of the council of state, and in 1853 general of division. In the first part of the Crimean War he was employed in the expedition to the Baltic Sea, and directed engineering operations against Botnarsund , but early in 1855 he was sent to the Crimea, where he succeeded General Bizot as chief of engineers. For some years he had been the most trusted militarj adviser of Napoleon III, and he was now empowered to advise the generals on the spot in accordance with the wishes of the sovereign and the home government.

This delicate and difficult task Niel managed to carry out with as much success as could be expected, and he had the credit of directing the siege operations at the Battle of Malakoff. His reward was the grand cross of the Légion d'honneur. From 1855 to 1859 he was employed at headquarters, and also served in the senate. In the war against the Austrians in the latter year Niel commanded the IV corps , and took a brilliant part in the battles of Magenta (June 4, 1859) and Solferino (June 21, 1859). On the field of battle of Solferino he was made a marshal of France.

After service for some years in a home command, he became minister of war (January 20, 1867 to August 13, 1869). In this capacity he drafted and began to carry out a far-reaching scheme of army reform, based on universal service and the automatic creation of large reserves, which needed only time to mature. He also rearmed the whole of the army with the chassepot rifle. But he did not live to complete the development of his system.

He died in Paris, and a year later the Franco-German War destroyed the old imperial army upon which the new formations were to have been grafted.