Someone bought the slip of paper with the winning numbers at a shop in La Coruna last year, but lost it before they got out the door.
A good samaritan found the ticket for the June 30, 2012, draw and handed it to the manager, who notified authorities.
City Hall posted a notice on the lost-and-found section of its website on Monday.
"I'll be the first Spanish mayor who's searching for a millionaire not to ask for money but to give it," La Coruna Mayor Carlos Negreira said in a statement.
Authorities won’t reveal where the ticket was bought or at what time the purchase was made to ensure they can reliably test the credibility of people claiming to be the legitimate owner.
Under Spanish law, La Coruna must search for the owner of the winning ticket for up to two years.
If the search fails, then the person who found the ticket, Reija Gonzalez, will receive the prize, the Guardian reported.