The body of Miguel Blesa, 69, was discovered today inside a car at a private estate in Villanueva del Rey in the southern province of Cordoba, a Guardia Civil police spokeswoman said.
Spanish media said his death appeared to be a suicide but police refused to confirm it, saying only that the authorities were investigating.
Spanish daily El Pais said Blesa was having breakfast with friends at the hunting estate and left the table to move his car. He was found dead shortly after, it added.
Blesa, one of the figureheads of Spain's financial crisis who had links to senior members of the governing Popular Party, was chairman of Caja Madrid until it merged with six other troubled savings banks to create Bankia.
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A court in February sentenced him to six years in jail for misusing the bank's corporate credit cards.
Rato, the one-time head of the International Monetary Fund who chaired Bankia between 2010 and 2012 when it was nationalised, was sentenced to four-and-a-half-years in prison over the same scandal.
In total, 65 former staff were found guilty of using corporate credit cards for irregular expenses such as buying jewels, paying for holidays or expensive clothes.
A total of 12 million euros is said to have been spent on the so-called "black cards".
The case sparked widespread anger when the scandal first broke in 2014, at a time when Spain was recovering from years of recession and a banking crisis partly triggered by Bankia's massive bailout.
Blesa was accused of setting up the scheme and was found guilty of having misappropriated 435,00 euros.
He was jailed as part of a probe into Caja Madrid's purchase in 2008 at an inflated price of the City National Bank of Florida, but was released 15 days alter after that case was dropped.
Blesa was in the middle of another lawsuit involving irregular bonuses during his time at Caja Madrid. He always denied any wrongdoing.
"This has affected my image, my honour and my family," he said last year during his trial for the "black cards" affair.