The Spanish government is meeting on Friday with banks to discuss a planned 3 billion euro ($3.06 billion) levy on lenders.
"We consider this extraordinary contribution as unfair. It was a complete surprise to us. We still need more explanation about the concept of the extraordinary benefits or profits that they are are supposed to cover," Jacobo Diaz told analysts in a call.
"Whatever additional taxes will be an additional tension in terms of capital ratios," he said.
Diaz was speaking after the lender published second quarter results, marked by a rise in lending income.
Last week, Spanish Economy minister Nadia Calvino said the rationale for taxing banks was to prevent windfall profits on expected interest rates hikes.
Senior bankers have told Reuters that higher borrowing costs should not be considered extraordinary gains after negative interest rates have weighed down their business over recent years.
($1 = 0.9812 euros)
(Reporting by Jesús Aguado; additional reporting by Emma Pinedo; editing by Barbara Lewis)