An initial stage of 16 groups of three teams will precede a knockout stage for the remaining 32 which will come into effect in time for the 2026 World Cup.
The sport's world governing body voted unanimously in favour of the change at a meeting in Zurich, Switzerland on Tuesday.
The number of tournament matches will rise to 80, from 64, but the eventual winners will still play only seven games during the tournament.
The tournament will be completed within 32 days - a measure to appease powerful European clubs, who objected to reform because of a crowded international schedule for football clubs.
The changes mark the first World Cup expansion since 1998.
FIFA sport's world governing body wants the World Cup to be more "inclusive for all" - and that means giving more of FIFA's 211 member associations the chance to take part in the tournament.