The musician has been with theatre producer Weisbrodt for over five years, but now plans to marry after New York officials allowed same-sex couples to wed.
The couple got engaged in November, but after the laws were changed last month, Wainwright revealed that they will now make proper wedding plans.
"I'm engaged, and me and my partner live in New York and they just passed gay marriage in New York and I was like, 'Goddamn it, now we really have to do this'," he told Lorraine Kelly.
Wainwright became a father to a daughter in February. He and Leonard Cohen's daughter Lorca Cohen are the birth parents, while Weisbrodt also acts as the child's father.
He will perform a five-night residency at London's Royal Opera House from July 18 to July 23.
Watch Rufus Wainwright perform 'World War 3' with Guy Chambers as part of BBC Two's Secrets of the Pop Song below: