An initial stage of 16 groups of three teams will precede a knockout stage for the remaining 32 when the change is made for the 2026 tournament.
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.
It will make a mockery of the qualification process for most confederations
Campaign group New Fifa Now
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.
The changes mark the first World Cup expansion since 1998.
New Fifa Now, a campaign group that says the governing body needs to reform, labelled the expansion “a money grab and power grab”.
“It will dilute the competitiveness of the tournament and, therefore, the enjoyment of fans,” it said in a statement.
“It will not help development of the game or provide improved competitive opportunities for lower-ranked nations. Instead, it will make a mockery of the qualification process for most confederations.”
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.
According to Fifa’s own research, revenue is predicted to increase to £5.29bn for a 48-team tournament, giving a potential profit rise of £521m.
History of World Cup overhauls
1 group of 4 and 3 groups of 3, with only top team progressing to semi-finals
Straight knockout tournament
15 (although only 13 took part)
3 groups of 4 and 1 group of 3, with top side progressing to final group of four
4 groups of 4, but only 2 games in each group, with top 2 sides through to quarter-finals
4 groups of 4, this time with 3 games. Top 2 sides through to quarter-finals
1974 West Germany
4 groups of 4 but now followed with 2 groups of 4, the 2 top sides competing the final
6 groups of 4 followed by 4 groups of 3, the winner of each qualifying for the semi-finals
6 groups of 4, top 2 sides and 4 best 3rd-placed teams qualifying for round of 16
8 groups of 4, top 2 sides progressing to knockout rounds