The musician played piano during McPhee's February 2008 wedding to her first husband, Nick Cokas.
You can see photos of Fosoter sitting at the keys behind the bride in PEOPLE's photos from the American Idol alum's first wedding.
Foster accompanied McPhee as she serenaded Cokas with Natalie Cole and Nat King Cole's hit "Unforgettable," after Cokas performed Billy Joel's "She's Got a Way."
McPhee confirmed the engagement news herself, while using a hilarious Ariana Grande gif, on social media. She added the 68-year-old popped the question during their vacation in Italy, which they've been public about on the inter respective Instagram platforms.
"He did it at the top of this mountain in Anacapri. Totally dark, only stars. Thankfully he didn't push me off the cliff. He said it was one or the other. And in the end he spared me," she joked via text.
Later Tuesday, Foster also confirmed the joyous news on social media, posting a photo of the lovebirds hugging while on a boat.
"🎼Yup!!," he captioned the snap — though daughters Erin, 35, and Sara, 37, had more to say about their new stepmom. "Mommmyyyy," Erin joked, while Sara wrote, "Out of the country. What did I miss?"
McPhee and Foster began dating in 2017 and while they have not spoken much about their relationship, they have been photographed together several times. In May 2018, the pair made their red carpet debut at the Met Gala in New York City.
This will be the fifth marriage for Foster, 68, and the second for McPhee. Foster was previously married to B.J. Cook, Rebecca Dyer, Linda Thompson and Yolanda Hadid, while McPhee married Nick Cokas in 2008, with her rep sharing in 2014 that the Smash star had filed for divorce.
"When Kat talks about David, her face lights up. She's really so happy. They're actually so sweet together," a source told Us Weekly in November 2017. "They aren't hiding it anymore... David is really charming. The age difference might seem substantial to some, but they are really cute together and it makes sense."1comments
In December 2017, Foster's daughter Sara Foster told Us Weekly that she and her siblings approve of their dad's relationship with McPhee.
"We totally approve!" she shared. "I mean, listen, we approve of whoever is going to be good to my dad at the end of the day and we're very honest about it. If we ever thought that anyone was in something for the wrong reasons, we would be very vocal about it. We just want someone that's going to treat him well and that cares about him, and I think he's with someone right now that really does."