McPhee's rep Leslie Sloane also confirmed the news. "What did I miss?" wrote Sara.
For Foster, this would be his fifth marriage. The couple has been together since late previous year.
Despite the couple's age difference, McPhee is well-liked by Foster's family, who apparently think the two are a fine match.
Foster and McPhee were first linked together last May, and by December of 2017, they couldn't seem to keep their hands off each other during a PDA-packed trip to Paris, France. Adding the age difference between 68-year-old David and 34-year-old Katharine works for them despite seeming odd to many. "And in the end he spared me", she said.
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McPhee also shared a post from the boat ride, slightly poking fun at her photographer.
We really, really like her, and we've known her for a really long time. David's children are daughters Allison, 48, Amy, 44, Sara, 37, Erin, 35, and Jordan, 31. "I love his outlook on life and how he treats people".
This would be McPhee's second marriage after her divorce from Cokas in 2016.
Both of Foster's daughters were close with their former step siblings, Gigi and Bella Hadid, when their father was previously married to Yolanda Hadid.