The 68-year-old musician popped the question to singer/actress Katharine, 34, during a romantic vacation in Capri, Italy in early July after just over a year of dating.
The 34-year age difference between the pair raised eyebrows when their relationship first began, but speaking for the first time about their romance, former "Smash" star Katharine insisted that age is no issue for them.
"It doesn't make sense to a lot of people, but it does to us," she told People magazine. "He has things to learn from me and I have a lot of things to learn from him, so I think we have a nice yin and yang."
David first met Katharine back in 2006 when he mentored on "American Idol", the TV competition she was appearing on. And since then, they have built up a strong friendship, which neither thought would develop into the relationship it did.
"The reality is that in the last 12 years I've always had a connection with her," David said, before Katharine added: "We genuinely love and respect each other and have for a very long time. There are just a lot of things when you look at the history of our relationship, neither of us could have written it. I would never have imagined that this would be our future together."
The marriage will be David's fifth. His ex-wives include Linda Thompson, B.J. Cook, and model Yolanda Hadid, who he divorced in October (17).
Meanwhile, Katharine wed Nick Cokas in 2008. They divorced in 2016.