Wilhelm Maria Theodor Ernst Richard Graf von Mirbach-Harff (2 July 1871 – 6 July 1918) was a German diplomat.
Born in Bad Ischl in Upper Austria into a Catholic Rhenan aristocratic family, he was a scion of Johann Wilhelm von Mirbach-Harff, founder of the Rhineland Knight academy. His parents were Ernst Graf von Mirbach and his wife Wilhelmine von Thun-Hohenstein (1851–1929).
From 1908 to 1911, Mirbach served as the embassy clerk in Saint Petersburg, and then as political councillor for the German military command in Bucharest. In 1915 he became the German ambassador in Greece, before being expelled from Athens in December 1916 when the Entente-leaning government of Eleftherios Venizelos took power.
He participated in the Russian-German negotiations in Brest-Litovsk from December 1917 to March 1918. He was appointed German ambassador to Russia in April 1918.
Mirbach was assassinated at the German embassy in Moscow by Yakov Grigorevich Blumkin and Nikolai Andreyev at the request of the Central Committee of the Left Socialist-Revolutionaries, who tried to incite a war between Russia and Germany. Blumkin entered Mirbach's residence in Moscow using forged papers and shot his victim at point blank range. As Mirbach tried to escape, Andreyev fired a second bullet and both of the assassins leapt out of the window and then drove away in a Cheka car. Mirbach's assassination signaled the beginning of the revolt of the Left Socialist-Revolutionaries in Moscow in 1918.
Mirbach was succeeded as German ambassador to Russia by Karl Helfferich.
Coincidentally, a later relative, Andreas von Mirbach, would be murdered by the Red Army Faction at the West German Embassy siege in Stockholm in 1975.