Wilhelm Aleksander Thesleff (27 July 1880, Viipurin maalaiskunta, Grand Duchy of Finland – 26 March 1941, Helsinki[1]) was a Finnish general, first Minister of Defence of Finland and briefly the commander in chief of the Finnish army.

He continued his military studies in St Petersburg, Nicholas General Staff Academy from 1904 to 1907, and the Officers Cavalry School from 1910 to 1911.

Thesleff served in the Imperial Russian Army during World War I from 1914 to 1917, until he was taken prisoner by the Germans in Riga in September 1917.

In March 1918 he was appointed as a liaison officer with the Baltic Sea Division during the Finnish Civil War.

After the resignation of Major General Wilkman, as a war minister, Thesleff became commander in chief of the Finnish military forces on 13 August 1918.