Police in Vasteras, a city 60 miles west of Stockholm, responded to a tip and found the unnamed man sitting in his car late Friday night opening Christmas cards, The Local.se reported. They searched the car and found the mailbox.
Posten, the Swedish postal service, puts out special red mailboxes in the runup to Christmas for holiday letters and cards.
Martina Saghamre, a spokeswoman for Vasteras police, said the box appeared to have been taken from a nearby store.
"He probably hoped that there would be money among the Christmas cards," she said. "Many elderly people send money for Christmas."
Saghamre suggested anyone giving cash find some way other than the mail. But the postal service said the theft of an entire box is unusual and even Christmas cards are rarely stolen.
"This is the seventh Christmas season I have worked and I've never heard of someone stealing an entire postbox," Posten spokesman Per Ljungberg said. "Overall it is very seldom that we have problems with theft of Christmas cards."
The cards found in the man's car are now on their way to the rightful recipients, police said.