Moscow said politics had played a role in the downgrade, the first by a major agency since Russia seized Crimea from Ukraine in March and leaving the S&P rating just one notch above junk status.
Foreign investors have been pulling money out of Russia since the country's economy hit the rails last year, a process that has intensified along with western concerns about Ukraine.
S&P said outflows might now speed up further.
"The tense geopolitical situation between Russia and Ukraine could see additional significant outflows of both foreign and domestic capital from the Russian economy and hence further undermine already weakening growth prospects," the agency said in a statement.
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