The 25-year-old has been advertising his club's car sponsor and has also appeared in adverts for petrol.
Reus also received speeding tickets on at least five occasions from 2011 before authorities realised he had been driving without a licence.
"The reasons I did it are something I cannot really understand," he said.
Dortmund said they had discussed the matter with the player and spokesman Sascha Fligge said: "He has promised such a thing will not happen again."
Reus, who missed Germany's World Cup triumph in July after picking up an ankle injury in the final warm-up game, has repeatedly been linked with a move to the Premier League but is currently out until the New Year with another ankle injury.
Borussia Dortmund and Germany winger Marco Reus has been fined more than half a million pounds for driving without a licence.