The boy was born Samantha to Nixon and her former beau, teacher Danny Mozes.
But now, the Sex and the City star and New York governor candidate, 52, has shared the trans news on Instagram on Friday about her boy, known as Seph.
She captioned a photo of her son on the Trans Day of Action with a photo from his graduation earlier this month from the University of Chicago.
"I'm so proud of my son Samuel Joseph Mozes (called Seph) who graduated college this month. I salute him and everyone else marking today's #TransDayofAction. #TDOA," Nixon wrote with the smiling photo of the two of them.
Nixon had Samuel, previously known as a girl named Samantha, with ex Mozes, and also son Charles, 15. The flamed-haired actress split from Mozes in 2003 and came out as gay.
She married galpal Chistine Marinoni in 2012. They have a son Max, 7.
In March, the SATC star surprised the state of New York by announcing that she was running for governor against fellow Democrat Andrew Cuomo.
"We want our government to work again. On health care, ending massive incarceration, fixing our broken subway," Nixon said in her candidacy announcement video. "We are sick of politicians who care more about headlines and power than they do about us."
Nixon is a big underdog in the governor's race but she's making waves, calling for the abolishment of ICE this week in the wake of the immigration controversy.