Gorbachev said he had bought a 49% share of Novaya Gazeta with Alexander Lebedev, a former spy and businessman turned deputy for the pro-Putin United Russia party.
"Together with my comrade Alexander Lebedev we acquired 49% of Novaya Gazeta," Gorbachev told Kommersant daily.
"The question arises: Will the paper preserve its face? Our answer is simple: The editorial has the controlling stake."
Gorbachev, a long-time fan of Novaya Gazeta, bought 10% of the paper while Lebedev bought 39%. Editorial staff will keep the remaining 51% of the paper's shares.
Novaya Gazeta has published scathing reports about the Kremlin's handling of the conflict in Chechnya, the attack on Mikhail Khodorkovsky, the former Yukos owner, as well as official corruption.
Gorbachev pledged not to use the newspaper as a political vehicle.
"We need to provide a pluralism of opinions and the reliability of its publications, and it must be reflective of public opinion in Russia," he said.
"We, as shareholders, will co-operate with the editorial collective and will not adapt it to our corporate needs."
Gorbachev, who is credited with helping to end the Cold War with the United States, was awarded the Nobel Peace Prize in 1990.
Gorbachev has praised Putin's attempt to unite Russia's Soviet past with the present, but has sometimes seemed uneasy about the Kremlin's drive to boost control over the media.
All three nationwide television channels are state-controlled, and a number of major newspapers have recently been acquired by Kremlin-friendly interests.
Izvestia daily was bought last year by the media arm of the state-controlled Gazprom natural gas monopoly.
Lebedev, who once worked as a KGB spy but turned to finance and then politics, said the purchase would help civil society.
"It is our contribution to building civil society," Lebedev said. "We will be, to the best of our ability, guarantors that the paper will fall to neither oligarchs nor the state."
Novaya Gazeta says it has a circulation of 171,500 in Moscow and 513,000 in Russia's regions.