Shareholders allege irregularities in the way its stock market listing was handled, according to their lawyers Cremades & Calvo-Sotelo.
Spain’s public prosecutor last week launched an anti-corruption probe into the bank.
The country’s caja savings banks are coming under renewed scrutiny amid the government’s request for a bailout of its banks.
Though not all of the cajas are struggling, Madrid’s bailout plan puts the spotlight on a disparate and, until recently, lightly-regulated cornerstone of the Spanish banking sector.
Seven former savings banks, already patched up with state aid, will be first in line to tap European rescue funds requested by the country, the IMF has said.
Regional parliaments in Valencia and Catalonia have launched inquiries into the wider sector in the last two weeks.
Meanwhile, Liberbank is attempting to merge with Ibercaja and Caja 3 – the latest in a long line of attempts to shore up ailing cajas through consolidation.