Standard and Poor's stripped Russia on Monday of its investment-grade rating, cutting it by one notch to 'BB+' into speculative territory with a warning of weak growth and banks.
"The downgrade reflects our view that Russia's monetary policy flexibility has become more limited and its economic growth prospects have weakened," the ratings agency said in a statement.
The plunge in oil prices and Western sanctions have pummelled the Russian economy in recent months, with the ruble collapsing in value last month.
Standard and Poor's is the first to move Russia down into the speculative or "junk" territory.
Moody's earlier this month cut Moscow's rating to 'Baa3', one level above junk, as did Fitch, which moved it to 'BBB-'.
Standard and Poor's now forecasts that Russia's oil-dependent economy will contract by 2.6 percent in 2015 due to the fall in crude prices and Western sanctions over Ukraine.
It projected that the Russian economy will expand by about 0.5 percent annually in 2015-2018, below the 2.4 percent it notched up over the previous four years.
The lower growth forecast "reflects a lack of external financing due to the introduction of economic sanctions and the sharp decline in oil prices," said the ratings agency.
"We also expect that declining domestic purchasing power as a result of exchange rate depreciation and rising inflation will likely hamper Russia's growth prospects," it added.
The rating agency also said "we believe that Russia's financial system is weakening and therefore limiting the Central Bank of Russia's ability to transmit monetary policy."
Standard and Poor's was not confident about the Russian government's ability to restore long-term growth.
"We do not currently expect that the government will be able to effectively tackle the long-standing structural obstacles to stronger economic growth over our 2015-2018 forecast horizon," it said.
Those problems include perceived corruption, the weak rule of law, the state's pervasive role in the economy, and the challenging business and investment climate, according to the ratings agency.
Standard and Poor's put the outlook on the BB+ rating as negative.