The 36-year-old Broadway star and the 70-year-old Canadian composer are expecting their first child together.
Sources confirmed the news to People magazine on Thursday.
According to the outlet, the couple were shopping for baby items in Montecito, Calif. this week.
Foster is already father to daughters Allison, 50, Amy, 46, Sara, 39, Erin, 38, and Jordan, 34, from previous relationships.
McPhee and Foster were wed during a secret ceremony at St. Yeghiche Church in London, England in June 2019.