Moody's Investors Service has lowered the ratings of some of the world's largest banks, including Bank of America, JP Morgan Chase, Citigroup and Goldman Sachs.
The ratings agency said the banks were downgraded because their long-term prospects for profitability and growth are shrinking.
A downgrade usually means that it becomes more costly for banks to raise money by selling debt.
Investors demand higher interest for riskier debt, which is what the downgrades represent.
Based on reporting by AP and dpa