Kaepernick's protest was denounced as "unpatriotic" and "unsportsmanlike" by some, including members of the football community. The New York Giants' Justin Pugh tweeted his reaction on Saturday.
But Kaepernick added that he wasn't concerned about any backlash to his decision.
"This is not something that I am going to run by anybody," he said. "I am not looking for approval. I have to stand up for people that are oppressed.
"If they take football away, my endorsements from me, I know that I stood up for what is right."
The 49ers issued a statement after the exhibition game, which saw San Francisco lose to Green Bay, 21-10.
"The national anthem is and always will be a special part of the pre-game ceremony. It is an opportunity to honour our country and reflect on the great liberties we are afforded as its citizens," the club said.
"In respecting such American principles as freedom of religion and freedom of expression, we recognise the right of an individual to choose and participate, or not, in our celebration of the national anthem."
There's nothing in NFL policy mandating that players must stand for the anthem, but that didn't stop many fans from weighing in on the player's protest on social media.
