Marsters, 47, recently proposed to longtime girlfriend Patricia Rahman, 24, a student of fashion design, in Trier, Germany, his rep tells PEOPLE.
Marsters, who is also a musician and played with the band Ghost of the Robot, met Rahman at a concert in Amsterdam. Of their first meeting, Marsters told PEOPLE in 2006, "I met her backstage and got her phone number, but then my jeans were cleaned out by the crew and they threw it out. I'd given her my number, but she took down one digit wrong. It took her six months to call all of the different combinations."
The couple plan to wed this fall. Marsters will next be seen in the new CBS series Hawaii 5-0.