The mother of Cristiano Ronaldo, the world’s highest-paid footballer, was stopped last month at Madrid airport with €55,000 (£40,400; $61,000) in her handbag, El Mundo newspaper reports.
It said that Dolores Aveiro, 61, was stopped by the civil guard while waiting for a flight home to Portugal.
She could not explain to officials why she needed so much cash, El Mundo said.
It said that the money will be held by officials until she pays a fine and fills in the correct documentation.
Spanish rules drawn up to prevent money laundering stipulate that the maximum amount travellers are permitted to take out of Spain without declaring the transfer is €10,000.
Anyone who breaks this rule is not guilty of a crime, El Mundo said, but an administrative offence.
Mrs Aveiro is reported to spend long periods of time with her multimillionaire son in his luxury mansion near Madrid.
She was reported to have been briefly held by border police at Barajas airport in the middle of last month and must now pay a €600 fine in order to get the cash back.
Ronaldo’s contract at Real Madrid earns him around €350,000 each week.