S&P also removed Omnicom from CreditWatch, saying its outlook is now stable.
The ratings agency said the downgrade was due to the risk the uncertain economy presents to Omnicom's short-term debt, including a $900 million convertible note that could be put to the company for redemption July 31.
Separately, S&P for the first time since August removed Interpublic from CreditWatch, signaling it's not contemplating another near-term credit downgrade.
S&P's long-term Interpublic outlook remains "negative," signaling the possible rating direction in the next two to three years. S&P earlier this month cut Interpublic debt to "junk" status.