Planning to bring cash with you when you enter Spain? There is a limit, and going over it without declaring the money can cost you far more than the inconvenience of filling out a form.
Not knowing the rule is not an excuse. Spanish law does not accept "I didn't know" as a defense, so it is worth knowing this before you travel.
Spain's law against money laundering and terrorist financing sets a maximum amount of cash you can carry without declaring it. If you are carrying more than that limit, you are required to notify the Treasury and follow the declaration procedure the law sets out.
If customs authorities stop you and find that you were carrying more than the limit without declaring it, the consequences are serious. All of the cash can be confiscated on the spot, and on top of that, you can face a sanctions process with a possible fine.
The safest approach is simple: if you are carrying a significant amount of cash into Spain, check the current declaration threshold before you travel and declare it if you are over that amount.
