The sport's world governing body voted unanimously in favour of the change at a meeting in Zurich on Tuesday.
The number of tournament matches will rise to 80, from 64, but the eventual winners will still play only seven games.
Why expand? Fifa president Gianni Infantino has been behind the move, saying the World Cup has to be "more inclusive".
Speaking at a sports conference in Dubai in December, Infantino said expansion will also benefit "the development of football all over the world".
He added: "There is nothing bigger in terms of boosting football in a country than participating in a World Cup."
Despite saying "the decision should not just be financially driven", Infantino did highlight the possible financial upsides.
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| Gianni Infantino |
FIFA’s ruling council on Tuesday unanimously approved an expansion of the World Cup to 48 teams in 2026, with a format of 16 groups of three nations. “The FIFA Council unanimously decided on a 48-team #WorldCup as of 2026: 16 groups of 3 teams,” a tweet from FIFA’s official account said. The decision marks a major coup for the body’s president Gianni Infantino who has made enlarging football’s showcase event the centrepiece of his young administration. The controversial proposal has faced criticism from some of the sport’s most powerful voices, including warnings that it would dilute the quality of play and overburden already exhausted players. Africa and Asia could be the big winners in a larger format with a rise in their number of places, currently at 5 and 4.5 respectively. But in order to smooth over scepticism about World Cup reform within UEFA, it is likely that Europe will also see its allotments rise above the current 13 places.