The 62-year-old actress, who shares two children with husband Christopher Guest, shared the transition news with her child's permission in the latest issue of AARP magazine -- as she talked about "old ideas that don't work anymore."
The magazine noted one of those "old ideas" was that gender is fixed and not fluid, with Curtis saying she and Guest "have watched in wonder and pride as our son became our daughter Ruby."
Added Curtis: "And she and her fiancé will get married next year at a wedding that I will officiate."
25-year-old Ruby is a computer gaming editor, the actress confirmed -- while Ruby's older sister, 34-year-old Annie, is a married dance instructor. Curtis told the publication she has no grandkids yet, "but I do hope to" in the future.
In the article, Curtis also opened up about being married to Guest since 1984 -- calling him her "one and only." She revealed that, on a recent wedding anniversary, she wrote him a song with the lyrics, "I feel safe when I drive up and see that you are hope."
"That's the long marriage. It's the safety of knowing his car is in the garage, that I'm not alone, and that he's here," she noted, saying the two have no "big delusions of grandeur" about their relationship.
Curtis will next be seen tackling another longtime relationship -- that between Laurie Strode and Michael Meyers -- in the upcoming "Halloween Kills" and its sequel "Halloween Ends." The movie is scheduled for an October 15, 2021 release.