The Dutch player was diagnosed during a routine medical check-up and will now undergo surgery.
"During a urological check on Wednesday a testicular tumour was diagnosed," the club said in a statement.
He is the second player in recent months who has required surgery to remove a tumour from their testicle. Borussia Dortmund striker Sebastien Haller had to have surgery and undergo further treatment after being diagnosed with malignant testicular cancer during a pre-season training camp.
"As bitter as this piece of news is at the moment we are full of hope and confidence that Jean-Paul will get healthy and return to our group as soon as possible," said Hertha sports director Fredi Bobic.
"Until his return he will get from us all possible support. We will be tight as Hertha family and wish him all the very best."