The "American Idol" alum posed in a red bikini during the trip and wrote, "Been having a blast in Mexico."
The red two-piece is from Andie Swim.
On Thursday, McPhee, 35, honored her husband for his 70th birthday.
"I’ve known this man for 13 years now! What was once a working relationship became a friendship and ultimately led us here," she captioned a series of Instagram photos of and with the Grammy winner. "Anyone who meets him talks about how charming he is, so much so that he eventually charmed his way into my heart. I’m so proud that he’s my husband! May God bless you with many many more years. Happy birthday baby! @davidfoster."
Despite the couple’s 34-year age gap, the pair began dating in the spring of 2017 but didn’t confirm their relationship until almost a year later when they attended the Met Gala together.
Foster previously admitted that the brunette bombshell didn’t quite catch his eye initially, but over time the duo became inseparable.
“She didn’t catch my eye on the first day because I was so busy trying to do my job,” Foster explained. "But the reality is that in the last 12 years I’ve always had a connection with her.”
Incidentally, Foster played piano at McPhee's 2008 wedding to Nick Cokas. She and Cokas divorced in 2014.
McPhee is Foster's fifth wife. His most recent bride was "Real Housewives of Beverly Hills" star Yolanda Hadid, from whom he separated in 2015 and divorced in 2017.
Fox News’ Julius Young and Mariah Haas contributed to this report.