German Police said Kasia Lenhardt was found dead in her Berlin apartment in the early hours of Tuesday.
Kasia and Boateng had been dating for over a year but their relationship ended on the rocks a few days ago.
Last week, the centre-back confirmed his breakup with his girlfriend, saying it was a regrettable decision.
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Jerome Boateng and his ex-girlfriend Kaizia
"As is known from the media, I ended the relationship with Kasia Lenhardt,” he wrote on Instagram.
"We will go our separate ways from now on. That is regrettable, but for my family and for me it is the only right one.
"I had to take this step and draw a line. I apologise to everyone I've hurt especially with my ex-girlfriend Rebecca and our children. I am disappointed in myself too. A man has to take responsibility and act in the interests of his family and I'm doing it now. I wish Kasia all the best.”
A statement from the police concerning Kasia’s death said it could possibly be suicide, as there was no indication of a third party.
"Yesterday at around 8.30 p.m. there was a police operation in Charlottenburg on suspicion of suicide,” the Police statement said, as quoted by German Publication Bild.
“A lifeless person was found in the home. There are no indications of third-party debt."
Meanwhile, Bayern Munich manager Hansi Flick has confirmed that Boateng has left the team’s camp at the Club World Cup in Qatar after the incident.
"Jerome came to my room and asked if he could go home. He will not be available to us until further notice,” Flick told reporters.