Parker also included a link to Nixon's campaign website, where she said people can donate.
Nixon thanked Parker for her support on Twitter.
"Thank you @SJP, it means so much," she tweeted, along with a heart emoji and screen shot of Parker's post.
CNN reported that Parker's support came a day after her spokeswoman, Ina Treciokas, told Page Six that Parker was looking forward to speaking with Nixon.
“Cynthia has been my friend and colleague since we were little girls. I look forward to talking to her about her New York state gubernatorial bid,” Parker said.
At the time, she did not say Parker endorsed her.
Related: Actress Cynthia Nixon announces run for governor of New York
Former “Sex and the City” costars Kristin Davis and Kim Cattrall had offered support the week Nixon announced her bid.
"I am so proud of Cynthia Nixon," Davis tweeted March 19. "No one cares more than she does about EVERY person getting a fair change (sic) and a good education. I know that she would be an excellent Governor."
"I'm excited to support someone I have know for 20 years to be one of the most intelligent and caring people in my life," Davis later tweeted.
When a fan asked Cattrall about Nixon’s decision, the actress said, “I support & respect any former colleague’s right to make their own career choices.”
In a video announcing her candidacy, Nixon said she is a "proud public school graduate and a prouder public school parent.
“I was given chances I just don't see for most of New York's kids today,” she said. “Our leaders are letting us down...
“We want our government to work again, on health care, ending mass incarceration, fixing our broken subway. We are sick of politicians who care more about headlines and power than they do about us. It can't just be business as usual anymore,” Nixon said.
Nixon is running against incumbent New York Gov. Andrew Cuomo in September’s Democratic primary.