NFL quarterback Colin Kaepernick should "find a country that works better for him" said Donald Trump after the player's national anthem protest.
The Republican presidential nominee weighed in on the San Francisco 49er's decision to sit during The Star-Spangled Banner in a pre-game ceremony.
Mr Kaepernick said he will continue to sit out the national anthem until he sees improvements in US race relations.
Mr Trump called the quarterback's controversial stand a "terrible thing."
"I think it's a terrible thing, and you know, maybe he should find a country that works better for him," Mr Trump told KIRO radio in Seattle. "Let him try. It won't happen."
Mr Kaepernick, 28, stirred controversy on Friday when he sat during the national anthem before the San Francisco 49ers played the Green Bay Packers in an exhibition game.
The National Football League (NFL) player refused to stand in protest at the oppression of people of colour in the US, he said.
Racial tensions are mounting in the US, where a string of recent police killings and subsequent revenge killings have sparked protests across the nation.
But his actions have ignited a debate about respect, patriotism and the right of free speech.
The response of Senior Airman Brian Kolfage, who lost three limbs in Iraq in 2004, has been shared more than 30,000 times on Facebook.
"Next time I hear the national anthem I'll be sure to stand for the both us since you feel that you've been oppressed. We live in America, the land where no one is truly oppressed in the grand scheme of things. If you want to see the real meaning of oppressed I suggest you enlist in the military and travel abroad to the Middle East where you will witness what oppression is. Where women are beaten and killed in honour if they are raped, where 6-year-old little girls are forced to marry men in their 60s, that's just a little taste of what oppression is."
The US national anthem dos and don'ts
- code is enshrined in law but not enforced
- stand and face the flag
- put right hand over heart
- military in uniform and veterans should salute
What should you do during the US anthem?
Basketball legend, actor and writer Kareem Abdul-Jabbar wrote in the Washington Post that the NFL player deserved plaudits for his patriotism.
"What should horrify Americans is not Kaepernick's choice to remain seated during the national anthem, but that nearly 50 years after [Muhammad] Ali was banned from boxing for his stance and Tommie Smith and John Carlos's raised fists caused public ostracisation and numerous death threats, we still need to call attention to the same racial inequities."
The White House said it disagreed with Mr Kaepernick's protest but that he had a right to express his views.
And the NFL world has been divided over it.
Jim Brown, former Cleveland Browns running back, said: "He's within his rights and he's telling the truth as he sees it and I am with him 100 percent."
But Drew Brees, New Orleans Saints quarterback, said he wholeheartedly disagreed.
"He can speak out about a very important issue. But there are plenty of other ways that you can do that in a peaceful manner that doesn't involve being disrespectful to the American flag."
An NFL spokesman said players were "encouraged but not required" to stand during the anthem.
Colin Kaepernick
- 28 years old, mixed race, adopted and raised by white parents in Wisconsin
- Joined the San Francisco 49ers in 2011
- Former starter who led the team to the NFL's title game, the Super Bowl, in 2013, which they lost to the Baltimore Ravens. Since demoted to back-up
- Has signature touchdown celebration known as kaepernicking, involving the flexing and kissing of his right bicep
- Set an NFL record for the most rushing yards by a quarterback (181) in the 2012 season play-offs