It's been just over a year since rumors surfaced that the two were an item.
The couple was first linked romantically in 2017, though their relationship dates back more than a decade.
"He did it at the top of this mountain in Anacapri".
In the comments below the pic, David's daughter Erin - who is one year older than her dad's new fiancée - responded to the photo, "Mommmyyy" along with three red heart emojis. McPhee recently tweeted photos from the couple's getaway to Italy.
A source close to the couple calls their union "cute" and says they are really "sweet together".
TMZ is reporting that the 68-year-old musician and songwriter popped the question during their European vacation.
That post received comments from Foster's daughters Erin, 35, and Sara, 37, with Erin joking "Mommmyyyy", and Sara chiming in with, "Out of the country". "If we ever thought that anyone was in something for the wrong reasons, we would be very vocal about it. I think it's so amusing that people are amused by my love life", she added.
Congratulations are in order for Katharine McPhee and David Foster!
It will be the fifth marriage for Foster - who was previously married to B.J. Cook, Rebecca Dyer, Linda Thompson and Yolanda Hadid - and the second for McPhee.