Nixon used the photo to celebrate Trans Day of Action during Pride month, sharing how proud she is of her son.
Seph, who was born Samantha Mozes in November 1996, is Nixon's first child with her then-partner Danny Mozes.
Trans Day of Action began in 2005, and is an annual rally and march held in late June in New York City. The two also share a 15-year-old son, Charles, while Nixon is also mom to 7-year-old son Max with her wife, Christine Marinoni.
The 52-year-old NY governor candidate shared a picture of her son celebrating the Trans Day of Action.
"We want our government to work again".
Although this is the first time that Nixon has publicly mentioned her son's gender identity, she has been an outspoken advocate for LGBTQ+ rights, both before and after announcing her run for NY governor.
Plus, former co-star Kim Cattrall is in her corner, supporting Nixon's political run.
"It's supposed to be here to keep Americans safe, but what it's turned into is frankly a terrorist organization of its own", said the "Sex and the City" star. Her comments regarding the president come after Trump signed an executive order to stop the separation of children from their parents at the border.