The ratings agency said the banks are linked to the sovereign, so Spain’s reduced creditworthiness “implies a weaker credit profile for Spanish banks”.
The move follows a cut to the Spanish government’s own credit rating to just above junk status earlier in June.
Among the downgrades was a cut to Banco Santander, which saw its long-term rating cut to Baa2 from A3. The bank’s UK arm is a standalone business.
On Monday, Spain asked for money to help support its banks after an independent audit last week found they needed up to 62bn euros (£50bn) in extra funding.
European authorities had already agreed to provide up to 100bn euros ahead of assessments of the banks’ needs.