McNichol, who has been living with partner Martie Allen for 20 years, decided to share a picture of the two of them with People magazine.
"She hopes that coming out can help kids who need support," her publicist told the mag. "She would like to help others who feel different."
The "Empty Nest" costar -- who as a teen won Emmy Awards in 1977 and 1979 for supporting actress in a drama, for her work as "Buddy," the youngest child on "Family" -- has since left acting entirely. She told People in a 1989 interview that while working on a movie in 1982, she faced: "Nonstop crying. Nonstop anxiety. I was always shaking like a leaf. Freaking out, crying, confused."
Though rumors had swirled about drug abuse or bipolar disorder -- then called manic depression -- she said in 1989 that it was simply an emotional breakdown, a delayed reaction to life as a child star. "From the time I was very young," she said at the time, "I was a professional, making money and assuming responsibilities. I didn't live the life of a child. I was living the life of a 30-year-old."
In 1992, McNichol was diagnosed with bipolar disorder and left "Empty Nest" to attend to her health, according to Biography.
She decided to come out about her sexuality now because she was "very sad" about kids being bullied, and wanted to be open about who she is, she told People. Meredith Baxter, who played her older sister Nancy on "Family," came out as a lesbian in December 2009.
McNichol, her rep said, is healthy and enjoys living a very private life. She turns 50 in September.
-- Christie D'Zurilla
Photos: Kristy McNichol on "Family" in 1976, top right, on "Empty Nest" in 1989, bottom right, and celebrating a good play with Kari Michaelson in a 1995 celebrity softball game. Credits: ABC, top right; NBC, bottom right; Spencer Weiner / Los Angeles Times, left