The Russian and Indian leaders pledged co-operation
Russia has announced it will build 16 nuclear reactors in India as part of defence and energy deals.
The long-anticipated nuclear agreement came as Russian Prime Minister Vladimir Putin visited India.
He said nuclear co-operation was one of the most important aspects in the partnership between the two countries which have strong trade ties.
Russia's state-owned nuclear company earlier said six of the reactors would be built by 2017.
Russia is already building two reactors in the southern Indian state of Tamil Nadu.
"The agreement sees construction of up to 16 nuclear reactors in three locations," Deputy Prime Minister Sergei Ivanov, accompanying Mr Putin, said.
Russia is competing with French and US firms for contracts to build nuclear power plants in Asia's third-largest economy which is looking to increase its energy supply to sustain rapid economic growth.
The increased competition began after India's landmark civilian nuclear deal with the US in 2005 which ended the isolation India had experienced since it tested an atom bomb in 1974.
"This is one of our most important and promising areas of co-operation," Mr Putin said in New Delhi on Friday.
Coal still accounts for more than 50% of India's energy use - but a substantial expansion of nuclear power reactors is proposed over the next few decades.
Over the next decade alone, the contribution of nuclear energy is expected to rise from just 3% to 6% of India's total needs.
The two countries have had close links since Soviet times, with India remaining a top buyer of Russian weaponry.
However, the relationship faces new challenges, including competition from the West and the growing economic and military might of China.
As part of the deals signed during Mr Putin's visit, Russia will refit an aircraft carrier, supply an additional 29 fighter aircraft to India and jointly develop transport aircraft.
Russian and Indian officials also discussed boosting bilateral trade, which is currently worth $8bn.