Japan defeated Russia in the Russo-Japanese War of 1904–1905. The conflict was fought mainly over competing ambitions in Korea and Manchuria, and Japan’s victory made it the first modern Asian power to defeat a major European empire in a large-scale war.
Japan launched a surprise attack on the Russian fleet at Port Arthur in February 1904, before a formal declaration of war. Japanese forces then won important land and naval battles, including the Battle of Mukden and the Battle of Tsushima. At Tsushima in May 1905, Admiral Tōgō Heihachirō’s fleet destroyed much of Russia’s Baltic Fleet after it had traveled around the world to reach East Asia.
The war ended with the Treaty of Portsmouth, negotiated with U.S. President Theodore Roosevelt’s mediation. Russia recognized Japan’s dominant position in Korea and transferred important rights in southern Manchuria, including control of Port Arthur and the South Manchurian Railway. Japan also gained southern Sakhalin.
Manchuria was a region, not an independent country participating as the victor, so it is not the answer. Korea and China were central to the dispute, but neither defeated Russia in this war.