The British bomb was found at a parking area near the west tribune of the Signal Iduna Park hours before coach Juergen Klopp's news conference ahead of the Ruhr valley derby against Schalke 04 on Saturday.
It was discovered after viewing aerial photographs, city officials said.
Efforts to defuse the bomb and render it safe would start later 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.
(Reporting by Karolos Grohmann, editing by Pritha Sarkar)
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