The new rule says players have to "stand and show respect for the flag," so raising a fist or linking arms could also constitute a violation.
If any player or team employee doesn't stand, the team will be fined at the league's discretion. Individual teams can also impose their own penalties or fines on their personnel.
The league didn't say how much the fine would be, or how "respect for the flag" would be defined.
NFL commissioner Roger Goodell said the vote was "unanimous" among owners, although San Francisco 49ers owner Jed York said he abstained.
Players do have an "out" though. They don't have to be on the field for the anthem. They can stay in the locker room without being penalized by the league.
Of course, many players probably don't see that as a compromise, since the whole point of protesting is to be SEEN protesting.
But then again, the TV networks will most likely point out who's NOT on the field . . . because let's face it, some people care more about the drama of what other people are doing rather than respecting the moment themselves.