The couple, who have been dating since 2014, are usually private about their relationship, but the 40-year-old actor has revealed that they really like group singing sessions with friends when they are not working, reported Grazia magazine.
"We karaoke. It's usually something in falsetto. I guess my Frank Sinatra is not bad because I'm kind of a baritone. But if I'm ambitious, I'll try Marvin Gaye or Rihanna.
"Rihanna went really badly - she was there," Fassbender said.
The actor also revealed he and his 29-year-old girlfriend have relocated to Lisbon, Portugal, and are enjoying the relaxed lifestyle there.