The 25-year-old actress praised her dad for "openly fighting" with depression and encouraging others to "fight on" against mental illness.
"Today is #WorldMentalHealthDay," wrote Zelda. "Mental illness is often misunderstood and misrepresented, but that's starting to change. Let's end the stigma.
Zelda and her father Robin in 2009
"Mental health IS as important as physical health and whether there are visible signs or not, the suffering is real," she added. "It can affect EVERYONE. So please, let's help stop the misconceptions and support those who need our help."
Robin Williams' daughter Zelda leaves Twitter and Instagram to 'heal'
Zelda went on to say encourage people to seek help for mental ilnesses, saying: "No matter what the misinformed say, you can't simply CHOOSE to make mental illness go away. It is NOT cowardly to suffer or seek help."
Lastly, my dad openly fought depression his whole life, both in general and his own. No matter what anyone says, it is a FIGHT. Fight on.— Zelda Williams (@zeldawilliams) October 10, 2014
The Detention star concluded by remembering her father's battle with depression.
Zelda posts emotional message for her late father
"Lastly, my dad openly fought depression his whole life, both in general and his own. No matter what anyone says, it is a FIGHT. Fight on."
Left to right: Robin Williams, Cody Williams, Marsha Garces and Zelda Williams in 1998
Robin Williams passed away in August.
The Oscar-winning actor committed suicide at the age of 63, leaving behind his wife of three years, Susan Schneider and his three children Zachary Pym, 31, Zelda Rae, 25, and 22-year-old Cody Alan Williams.