Spanish banks' bad loan ratio rose to 11.2 percent in May from 10.87 percent in April, according to data released on Thursday by the Bank of Spain.
Loans that fell in to arrears increased by 3.14 billion euros (4.11 billion U.S. dollars) since April to 170.2 billion euros in May.
In annual terms, the bank said bad loans increased by 14.2 billion euros compared to May 2012.
Spain's total credit portfolio fell from 1.54 trillion euros in April to 1.52 trillion euros in May, while experiencing a 12.7 percent decrease in comparison with May 2012 when Spanish total credit portfolio reached 1.74 trillion euros.
The non-performing loan rate peaked at 11.38 percent in November 2012 but decreased in December and February 2013 thanks to the Spanish bad bank (SAREB).
SAREB took toxic assets from those entities needing the European Union's financial help, reducing a big proportion of assets from the real estate sector of Spain's housing bubble that burst in 2008.
Experts expect the non-performing loan rate will increase in the following months as unemployment is not expected to decrease in the medium term.