A source confirmed the happy news to ET on Thursday.
"Katharine and David are expecting their first child together and the couple is very excited," the source tells ET. "Katharine and David discussed having kids early on and were on the same page."
People was first to report the news. In pictures published by the outlet of the couple in Montecito, California, on Tuesday, she sports what appears to be a noticeable baby bump.
Reps for Foster and McPhee did not reply to requests for comment.
In a YouTube video on Oct. 3 of McPhee singing Sara Bareilles' "She Used to Be Mine," she gave a big clue that she was pregnant, holding her stomach while singing the lyrics, "And then she'll get stuck / And be scared of the life that's inside her / Growing stronger each day 'til it finally reminds her / To fight just a little, to bring back the fire in her eyes / That's been gone, but used to be mine / Used to be mine."
This is 36-year-old McPhee's first child, while 70-year-old Foster has five daughters -- 34-year-old Jordan, 38-year-old Erin, 39-year-old Sara, 46-year-old Amy and 50-year-old Allison -- from previous relationships.
The former Smash star and the legendary record producer got married in June 2019. It's the second marriage for McPhee and the fifth for Foster. McPhee was previously married to Nick Cokas from 2008 to 2016, while Foster was previously married to B.J. Cook, Rebecca Dyer, Linda Thompson and Yolanda Hadid.
McPhee Instagrammed on her wedding day, "Exactly 13 years ago today my very first single, 'Somewhere Over The Rainbow,' was released right after Idol. Today... I’m marrying the man who produced it. Life is full of beautiful coincidences, isn’t it? Thank you for taking me over the rainbow, David. ❤️"