Prescott's Instagram post represents his first comments on social media regarding George Floyd, who died while in police custody in Minneapolis last week.
"As our communities take action, protesting and fighting for the justice of George and every black life, I am with you!" Prescott wrote.
Prescott said he has battled anxiety while facing the coronavirus pandemic without football. He also is mourning the death of one of his brothers, Jace, in April.
"He and I shared the same mission: Find a bigger purpose," Prescott wrote. "As I process the passing of my brother, I have come to realize we are not given a voice to pronounce how much we matter. It is our obligation to tell our neighbor how much they matter to us and take a stand for the greater good of each other."
Born to a black father and Native American and white mother, Prescott said being multiracial "is beautiful and that is what this country is."
Floyd, who was black, died after Derek Chauvin, a white police officer, kneeled on his neck for more than eight minutes. Chauvin and three other officers subsequently were fired, and Chauvin was charged Friday with third-degree murder and second-degree manslaughter.
On Wednesday, Chauvin's charge was upgraded to unintentional second-degree murder. The other three officers -- Thomas Lane, J. Kueng and Tou Thao -- have been charged with aiding and abetting second-degree murder and second-degree manslaughter, according to criminal complaints.
Floyd's death sparked protests across the United States.
While expressing respect for police, Prescott also asked them to change.
"I have the utmost respect for those of you with a passion for protecting and serving your communities," Prescott wrote. "When you chose to wear the badge of a police officer, you pledged to PROTECT life and property through the enforcement of our laws and regulations. How can you claim to uphold the law when those within your ranks don't abide by it? You need to hold your own accountable. Each of you are as guilty as the men who stood beside Derek Chauvin if you do not stand up against the systemic racism plaguing our police forces nationwide. TAKE ACTION!"
Prescott said he would stand alongside the police in the reform.
"We will clean our streets and our communities not only of the looting and violence," he wrote, "but most importantly the racism, racial-profiling and hate!"
The Associated Press contributed to this report.