Sidney George Reilly (; – 5 November 1925) — known as "Ace of Spies" — was a Russian-born adventurer and secret agent employed by Scotland Yard's Special Branch and later by the Foreign Section of the British Secret Service Bureau, the precursor to the modern British Secret Intelligence Service (MI6/SIS). He is alleged to have spied for at least four different great powers, and documentary evidence indicates that he was involved in espionage activities in 1890s London among Russian émigré circles, in Manchuria on the eve of the Russo-Japanese War (1904–05), and in an abortive 1918 "coup d'état" against Vladimir Lenin's Bolshevik government in Moscow.