Fifa president Gianni Infantino had voiced support for the law change in an interview with British broadcaster Sky News last month.
“If a player covers his mouth and says something, and this has a racist consequence, then he has to be sent off, obviously,” Infantino said.
“There must be a presumption that he has said something he shouldn’t have said, otherwise he wouldn’t have had to cover his mouth.
“If you do not have something to hide, you don’t hide your mouth when you say something. That’s it, as simple as that.”
In a separate law change announced on Tuesday to be enforced at the World Cup, Fifa said that red cards would also be introduced for players leaving the field of play in protest at a referee’s decision.
“At the discretion of the competition organiser, the referee may sanction with a red card any player who leaves the field of play in protest at a referee’s decision,” FIFA said.
“This new rule will also apply to any team official who incites players to leave the field of play.”
Fifa said a team causing a game to be abandoned will forfeit the match.
The move follows the uproar at this year’s final of the Africa Cup of Nations, when Senegal’s players, head coach Pape Thiaw and his staff walked off the pitch in Rabat after Morocco were awarded a penalty in added time, which forward Brahim Diaz ultimately missed.
Senegal went on to win the final 1-0 in extra time, but were sensationally stripped of the title by the Confederation of African Football (CAF) in a bombshell decision issued last month.
The law changes came as FIFA delegates gathered in Vancouver before Thursday’s FIFA Congress, the final gathering of football’s global governing body before the World Cup gets underway in Canada, Mexico and the United States in June.