"Protests and political instability are in our view likely to negatively affect economic performance and depress future growth prospects," especially for Bahrain in its role as a financial hub, S&P said in a statement from London today.
"There remain questions as to whether or not GCC economic aid will be able to offset these political and reputational risks."
The Gulf Cooperation Council, led by Saudi Arabia, on March 10 announced $10 billion (Dh36.7bn) each in aid for both Bahrain and Oman, two of its six members.
Protesters in the two countries are demanding free elections as well as more housing and jobs.
S&P downgraded Bahrain's main credit rating by two levels to BBB on March 18. On March 7, it placed Oman's A-rating on review.