"Did a thing. Another ring. She said YES," Stewart captioned photos of the proposal that she shared on Instagram.
Stewart, a two-time WNBA champion, was the regular-season MVP and the WNBA Finals MVP in 2018 as well as the Finals MVP last year.
Xargay, 30, is from Spain, and she previously played for the Mercury in 2015 and 2016. She is currently on Phoenix's preseason roster.
Stewart, 26, also is coming off being named MVP for the EuroLeague and the Russian League, winning championships in both last month with UMMC Ekaterinburg.
A four-time NCAA champion at UConn, Stewart is entering her fifth season in the WNBA. She missed 2019 with an Achilles tendon injury.