The 25-year-old, who has been involved in advertising for the club's car sponsor and a petrol company, received speeding tickets at least five times since 2011 before the authorities realised he had been driving without a licence ever since.
"I decided back then to take that road but the reasons I did it are something I cannot really understand today," Reus, a transfer target for several top European clubs, was quoted as saying by the Bild newspaper.
"Today I know that I was too naive and that it was stupid. I have learned my lesson and this will not happen again," he said.
Dortmund's prosecutor's office said the 540,000 euros (425,662.6 pounds) fine related to incidents from 2011 until this March.
Dortmund coach Juergen Klopp said Reus had admitted making a big mistake when he took a "wrong turn in his life.
"I was also surprised. I talked to him today and he has fully accepted it was a mistake," he told reporters.
"He took a wrong turn at some point. But everything is back to normal now. He is a great guy who made a mistake."
Klopp said it may not have been easy for Reus to go back and take a test while already being so well known as a Germany international. "He probably saw no way out of this," he added.
The club said they had discussed the matter with the player who said he had made a "stupid" mistake.
"He has promised such a thing will not happen again," club spokesman Sascha Fligge told reporters. "We see no reason to doubt his words because we hold Marco both as a player and as a person in high regard."
Reus, who missed Germany's World Cup triumph in July after picking up an injury in the final warm-up game, is currently out of action with another ankle injury.
(Reporting by Karolos Grohmann; Editing by Justin Palmer and Ken Ferris)
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