Among the banks downgraded Friday were some of Italy's largest, including UniCredit SpA, Intesa Sanpaolo SpA and Banca Monte dei Paschi di Siena SpA.
The agency also lowered its Banking Industry Country Risk Assessment for Italy to "4'' from "3." The so-called BICRA rating is on a 1-to-10 scale, with "1'' representing countries with the lowest-risk banking systems.
S&P's action came after the agency downgraded the credit rating on Italy's government debt by two notches last month. It was among nine countries that use the euro that were downgraded by S&P in January.