Dallas Cowboys quarterback Dak Prescott has pledged $1m to "improve police training and address systemic racism" in the United States.
It comes in the wake of the death of George Floyd, an unarmed black man, while being restrained by a white Minneapolis police officer on 25 May.
Protests have been held across the United States as well as globally.
"As a black multiracial American, I am disgusted and unsettled," Prescott, 26, on Wednesday.
"I try to understand and find the positive in every situation or aspect of my life.
"As our communities take action, protesting and fighting for the justice of George Floyd and every black life, I am with you!"
Prescott said he had the "utmost respect" for the police, but added: "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."
Chauvin, the police officer who kneeled on Floyd's neck for almost nine minutes to pin him down, has since been charged with murder and been sacked.
On Wednesday, the other three officers involved were charged with aiding and abetting murder.
Meanwhile, New Orleans Saints quarterback Drew Brees said he would not support renewed NFL kneeling protests, saying to do so would be "disrespecting the flag" of the United States.
Since Floyd's death sparked protests across the US, athletes have voiced support for former NFL star Colin Kaepernick, who in 2016 began kneeling during the US national anthem.
Former San Francisco 49ers star Kaepernick, who said he was refusing to stand in order to draw attention to racial injustice, was later frozen out of the league and has been unemployed since being released in 2017.
US President Donald Trump said players who 'took a knee' should be fired.
"I will never agree with anybody disrespecting the flag of the United States of America or our country," Brees told Yahoo.
"Is everything right with our country right now? No, it is not. We still have a long way to go.
"But I think what you do by standing there and showing respect to the flag with your hand over your heart, is it shows unity."
Brees is the NFL's all-time passing yardage leader and one of the biggest names in the sport.
His remarks came as Seattle Seahawks coach Pete Carroll praised Kaepernick's original protests.
Carroll said society "owes a tremendous amount" to Kaepernick for taking a stand against police brutality and racial injustice.
"He stood up for something he believed in - and what an extraordinary moment it was that he was willing to take," Carroll told The Ringer's 'Flying Coach' podcast.
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