"Very important that NFL players STAND tomorrow, and always, for the playing of our National Anthem," Trump posted on Twitter, adding "Respect our Flag and our Country!"
Later Trump tweeted an embedded video of fans and players standing at an NHL game, writing "19,000 RESPECTING our National Anthem!" Other Twitter users posted that the video Trump put up was nearly a year old.
The symbolic gesture of protest during the traditional playing of "The Star Spangled Banner" was adopted by some African-American players over the last year to protest against racial disparities in the criminal justice system.
On Tuesday Trump called on the league to ban players from kneeling in protest at games while the national anthem is played, tweeting "The NFL has all sorts of rules and regulations. The only way out for them is to set a rule that you can't kneel during our National Anthem!" Trump wrote.
And on Thursday he continued the crusade, drawing a rebuke from the NFL after he said football team owners are afraid of their players.
The Republican president told "Fox & Friends" in an interview that he is friends with many NFL team owners and they were "in a box" over how to handle the kneeling protests of racial disparities in the country.
"They say, 'We are in a situation where we have to do something.' I think they're afraid of their players, you want to know the truth. And I think it's disgraceful," he said. Trump did not elaborate.
The NFL rejected the president's remarks as not factual.
Trump also called, again via Twitter, for fans to boycott NFL games.
A week ago coaches, support staff and even some owners joined team members in silent support by kneeling, linking arms or staying off the field during the anthem.
The players' protests go back to last year's football season, when Colin Kaepernick, a San Francisco 49ers quarterback, protested police treatment of African-Americans by not standing during the anthem.
The protests culminated a week ago when scores of players, following calls by the president to fire protesting athletes, sat or knelt as the anthem was played.
(Reporting by Chris Michaud; Editing by Christopher Cushing)