Communist Sergei Levchenko was elected governor of Siberia's Irkutsk region, results showed on Sunday, the first politician from a nominal opposition party to win a vote for regional boss since the Kremlin restored such elections in 2012.
Levchenko, a Communist member of the State Duma lower house of parliament in Moscow, won 56.9 percent of the vote, easily defeating the incumbent, Sergei Yeroshenko, from United Russia, President Vladimir Putin's political party.
The positions of governor of Russia's 85 regions are among the most powerful in the country. From 2005 until 2012 they were appointed by the Kremlin, and since then United Russia candidates or Kremlin supporters have won all contests.
Campaign manager Yevgeny Rulkov, who is also a Communist party Duma member, said Levchenko's win was now certain.
"There is no doubt, it's a final conclusive victory," he said. "We worked for this victory and we achieved it."
Candidates from United Russia or other candidates loyal to Putin swept the board in local elections earlier in September, which the Kremlin had cast as a dry-run for a nationwide parliamentary election next year.
The Communists are Russia's largest nominal opposition party, although they strongly support the Kremlin-line on contentious international issues like last year's annexation of Ukraine's Crimea region.
Opposition figures who are more critical of the Kremlin, like anti-corruption activist Alexei Navalny, say the authorities tolerate the Communists to divert attention from real opponents.
Some voters in Irkutsk said they supported Levchenko because of dissatisfaction with the United Russia candidate, at a time when lower oil prices and Western sanctions over the Ukraine crisis have pushed Russia's economy into recession.
Rulkov said future election wins for Russia's opposition parties would be hard to come by.
"It is not all as easy as it seems after the victory of our candidate," he said. "Future victories for opposition candidates will be achieved with sweat and blood."