The redheaded star took to social media to post a photo of herself wearing a shirt that reads "We'll See You Tomorrow" and told fans:
"Today is World Suicide Prevention Day. Check in on loved ones, listen to what they say, how they say it and show them kindness and support. There is strength in numbers. And if you're feeling down, please reach out. Your life is worth fighting for. Remember, you are fully and unconditionally loved, xxjess."
Chastain has been a longtime advocate for TWLOHA and has only recently spoken out about her younger sister's suicide in an interview with InStyle late last year.
"It completely changed the person I am," she told the magazine when asked how the family tragedy affected her. "A movie, Oscars, a dress, if someone thinks I'm stupid…I realized nothing is that important."
The 38-year-old also shared how she hopes to carry on Robin Williams' legacy after she attended the Juilliard School on a scholarship provided by the late comedic legend.
"I always thought in some way I would have a chance to pay him back," she shared. "It was a very strange thing to have never met him. I didn't want to be stalkerish, but then you always question yourself: Did I do enough? I'm hoping there's a way to continue his legacy and keep the scholarship going."
If you or someone you know needs help, please call the National Suicide Prevention Lifeline at 1-800-273-TALK (8255)