The British bomb was discovered at a parking area near the west tribune of the stadium hours before manager Jurgen Klopp’s press conference ahead of the Ruhr valley derby against Schalke on Saturday.
It was discovered after viewing aerial photographs, city officials said, and efforts to defuse the bomb and render it safe started on Thursday afternoon.
“It is not yet known how long the stadium area will remain off limits,” Dortmund, who shut their Fan World space and club museum, said in a brief statement.
Unexploded World War II ordnance is a relatively common occurrence in Germany where several million tonnes of bombs were dropped during the war by Allied and Soviet forces.
The city of Dortmund suffered extensive destruction during bombing raids.