Russia's dollar-denominated RTS stock exchange index dropped 2.87 percent compared to the previous close, while the country's ruble-denominated MICEX index fell 0.13 percent at closing on Tuesday.
The Russian stock market joined the panic on international trading floors on Friday when S&P took the decision to cut the U.S. top rating by one notch to AA-plus over concerns of America's growing budget deficit.
Analysts have said the market would watch the results of the Federal Reserve meeting. The Fed will deliver its policy statement at around 19:15 GMT.
Many experts expect the Fed to continue pumping more money in the economy through the third round of quantitative easing QE3.
"Investors are seriously counting on the Federal Reserve to at least hint at possible additional measures to support the economy, and given the current situation on the financial markets, the third quantitative easing program QE3 could be announced," Aton analyst Yelena Kozhukhova said.
"But even in this case it would be too early to count on even a short-term positive reaction. Investors are likely to want decisive moves not only from the Fed, but also from other regulators to regain confidence in the economy."
