The socialite has been dating The Hills star for several months, following her November split from rocker Benji Madden.
She was forced to deny recent reports suggesting the reality TV hunk had popped the question on a dinner date.
But Hilton insists she will wed Reinhardt one day, telling E! Online, "He's going to be my husband." She adds, "We're best friends. It's not like we just met. We've known each other over the past year. I was in a relationship before and we reconnected. I'm really in love and really happy."