Italy's Banca Monte dei Paschi di Siena reported a loss of 47.4 million euros ($60.3 million) in the third quarter and of 1.664 billion euros for the first nine months of the year.
Analysts polled by Dow Jones had forecast a profit of 75.9 million euros and the bank posted a profit of 42.2 million euros in the third quarter last year.
The world's oldest surviving lender, the Tuscany-based bank has been critically exposed to the eurozone debt crisis and was forced to accept a government bailout in June to pay off debt and shore up its capital.
Moody's credit rating agency last month downgraded the bank to "junk" status on worries that recapitalisation plans will prove insufficient. The bank has also said it will reduce its workforce by 4,600 people by 2015.
Banca Monte dei Paschi di Siena, which was founded in 1472, is one of four European banks that failed to pass European Banking Authority stress tests.