Denver Broncos linebacker Brandon Marshall took a knee during the United States national anthem on Thursday.
Marshall was showing solidarity with San Francisco 49ers quarterback Colin Kaepernick, who chose not to stand during the national anthem before preseason games as a form of protest.
Kaepernick and Marshall were team-mates at the University of Nevada.
"While we encourage members of our organisation to stand during the national anthem, we understand and respect it being a personal decision," the Broncos said in a statement.
Kaepernick is protesting for what he says is the oppression "of black people and people of colour".
He says he will continue to sit until significant change in how police officers treat minorities occurs.
His stance has gained momentum with 49ers team-mate Eric Reid, Seattle Seahawks cornerback Jeremy Lane and United States midfielder Megan Rapinoe all kneeling before recent games.