Helmut Seifert, 47, an ethnic German originally from Russia, was enraged when he heard his daughter was having a relationship with the much older Phillip Genscher, the Telegraph reports.
Seifert claims to have approached police in his hometown of Bielefeld but officers said they were powerless to intervene, the Telegraph reports
"You said you couldn't stop him - so I did," Seifert said. "I saw it as my duty as a father."
Seifert, a factory worker, recruited two work colleagues, went to Genscher's house, forced the older man to take off his pants and then castrated him while fully conscious, the Telegraph reports.
"The severed testicles were taken away by the perpetrator," the police said in a statement.
The victim barely survived the incident, but did not have all his parts reattached, the Telegraph reports.
Seifert pleaded guilty and will be on trial for attempted murder next year, but has remained silent on who his accomplices were, the Telegraph reports.