The club's CEO Hans-Joachim Watzke has confirmed that the Champions League finalists are working on bringing in one more player to Signal Iduna Park this summer
Jurgen Klopp's squad has already been bolstered by attackers Henrikh Mkhitaryan and Pierre-Emerick Aubameyang and defender Sokratis Papastathopoulos this summer.
However, the Ruhr giants are not finished with their recruitment drive and are currently targeting an unnamed foreigner.
"I could tell you a lot, but I'll be careful not to do too much," he told Spox.
"He's not coming from the Bundesliga and we are working to complete the transfer."
Dortmund kick off their season with the DFL-Supercup curtain-raiser against Bayern Munich at Signal Iduna Park on July 27.