The couple is currently on vacation in Europe, which is where the proposal is believed to have taken place. On Sunday, sources say McPhee was showing her engagement ring to friends and family over FaceTime while the couple was in Capri, Italy.
McPhee has been sharing several shots from the trip, including a photo of herself sitting on a boat, leaning towards the camera as a dog lounged behind her, the Italian flag visible on the back of the craft.
"If only you saw the first 50+ pics it took to get to these two," the 34-year-old wrote, tagging Foster as the photographer.
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.
McPhee and Foster have been dating since last year 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.
"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."
Last week, the duo attended the Argento Ball for the Elton John AIDS Foundation in Windsor, England, before heading to Italy.
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."
Photo Credit: Getty / John Sciulli