The rule requires businesses to prominently display posters that explain the right to bargain collectively, distribute union literature and engage in other union activities without reprisal.
Union advocates say the rule gives workers information they should know about their legal rights. But business groups say they fear the posters will make it appear the government is encouraging workers to join unions.
The board under the Obama administration has taken a number of steps favorable to unions, which are trying to reverse years of membership declines.
The new rule exempts some small businesses. It takes effect Nov. 14.