It cut all of BP's long-term debt securities by three notches to "A2" from "Aa2."
It is the second time this month that Moody's has downgraded the company and follows downgrades from other ratings agencies as well.
Moody's cited the worsening impact of the oil spill and said the catastrophe will hurt BP's finances for years to come.
The ratings agency noted that BP's funding of a $20 billion spill fund doesn't limit its impact for the spill.
Moody's is reviewing the ratings and could downgrade them again. Moody's said it is continuing to monitor the company's efforts to cap the well, the environmental and economic impact of the spill, legal investigations and details of BP's liability for the $20 billion fund.
Lower debt ratings can make it more expensive and more difficult for a company to raise money, much like a personal credit score.
BP is still an investment grade company. BP did not immediately return a call early Friday morning for comment.
Company shares rose 75 cents to $32.46 before the market opened, about half what they were worth before the spill.