Moscow was barred from the international body five years ago over its annexation of Crimea - and its return has provoked fury from Ukraine.
The decision, originally scheduled for 8pm on Monday, was only taken in the early hours of Tuesday after 68 speakers queued up to speak and 200 amendments were tabled. All but one minor amendment failed to pass, and Russia was readmitted by 118 votes in favour and 62 against, with 10 abstentions.
The news came less than a week after a team of investigators connected the Kremlin to the events that led to the downing of Malaysian Airlines 17 over eastern Ukraine in 2014.
Russia had threatened to quit the Council of Europe's assembly altogether if it was not allowed to vote in Wednesday's election to elect a new general secretary of the body. The election is to replace Norwegian Thorbjørn Jagland, who has headed the organisation since 2009. It has also frozen its considerable 33 million euro annual budget payments to the Council of Europe since 2017.
Russia's departure from the Council of Europe would have meant Russian citizens losing access to the European Court of Human Rights, which is a Council of Europe body. The ECHR has proved to be a court of last resort for many Russians unable to receive justice at home.
Last week, French president Emmanuel Macron, one of the main lobbyists for the change, said non-membership harmed Russian citizens, and not its government.
“We want to ensure they have the opportunity to defend their rights,” he said.
The Ukranian delegation walked out of the assembly in protest at the decision. Dmytro Kuleba, Ukraine’s ambassador to the Council of Europe, had said ahead of the vote that approval of Russia “would be the unilateral surrender of the Council of Europe to Russian demands”.
The Kremlin, on the other hand, welcomed the move.
"This is a very positive thing," Dmitry Peskov, spokesperson for Vladimir Putin told reporters on a conference call on Tuesday. "We can only be positive about it."
"This is not about diplomatic victory for Moscow but for common sense. Parliament assembly can't work without the contribution of Russian delegation."
Provocatively, the Russian government immediately nominated Leonid Slutsky, chairman of the Duma's foreign affairs committee, to be vice chair of the Council. Mr Slutsky is still included in EU sanctions brought in response to the Russian annexation of Crimea.
Angela Merkel and Emmanuel Macron were in favour of lifting restrictions on Russia. Ukrainian President Volodymyr Zelensky expressed “disappointment” after the vote: “I tried to convince Mr Macron and Mrs Merkel that the return of the Russian delegation to PACE is possible only after Russia fulfils the assembly’s main demands,“ he said in a post on Facebook. “It’s a pity that our European partners did not listen to us.”
He said he was hopeful that proponents of the change would now insist on the release of the Ukrainian servicemen captured in December 2018 in clashes with Russian armed forces near the Kerch Strait.
Speaking in the House of Commons ahead of the vote, Theresa May said: "There has been this difference in the position of Russia in the Council of Europe.
"They have not been paying their contributions to the Council of Europe, but there is an issue that their membership of that body is one of the few ways available to the international community to hold Russia to account for its human rights violations."