Dortmund announced earlier this week that Haller will be out for an unspecified number of months after successfully undergoing surgery on the tumor.
Haller complained of feeling unwell in training last Monday and subsequent tests revealed the unfortunate news.
The 28-year-old is yet to make a competitive appearance for Dortmund after joining from Ajax Amsterdam in a €36m deal earlier this month.
The Ivory Coast international has been signed as a replacement for Erling Haaland, who has joined Manchester City, but Dortmund will have to wait a while longer before seeing their new striker in action.
A statement on the club's website read: "Sebastien Haller (28) will be absent from Borussia Dortmund for several months. The histological findings revealed a malignant testicular tumour.
"Haller therefore has to undergo chemotherapy treatment."
Dortmund's sporting director Sebastian Kehl added: "Sebastien will now receive the best possible treatment. The chances of recovery are very good. We wish him and his family a lot of strength and optimism and our thoughts are with him during this difficult time.
"We ask the media and fans for their understanding that we will not be releasing any medical details regarding the treatment of Sebastien Haller beyond the information mentioned, neither today nor in the coming months."
Haller was the third top scorer in the Champions League last season with 11 goals, behind only Robert Lewandowski(13) and Karim Benzema(15).
The former Eintracht Frankfurt and West Ham United frontman struck 47 goals and provided 16 assists in 65 appearances across all competitions for Ajax after joining in January 2021, winning two Eredivisie titles and two Dutch Cups.
