Haller underwent surgery last week after the tumour was discovered during a training camp but the club said the histological findings revealed a malignant testicular tumour which require him to undergo chemotherapy.
"Sebastien will now receive the best possible treatment. The chances of recovery are very good," Dortmund's sporting director Sebastian Kehl said in a statement.
"We wish him and his family a lot of strength and optimism and our thoughts are with him during this difficult time."
Haller was signed for an initial fee of €31 million earlier this month after the striker scored 34 goals last season for Ajax -- including 21 in the league and 11 in the Champions League.