A maintenance worker on Madrid's underground railway network stumbled upon the valuable cheque on Wednesday.
It was found after a train on line 5 of the Madrid Metro was pulled out of service because carriage doors weren't closing properly.
A member the train's repair crew then discovered the cause of the problem: a brown leather wallet jammed into the frame of a carriage door.
Inside were a Californian drivers licence, several credit cards — and the $2 million cheque.
All of these items have been handed in to Madrid's Sol police station.