"The revised format mitigates the risk of collusion and ensures that all the teams play a minimum of three matches, while providing balanced rest time between competing teams," FIFA said of the format for the first World Cup to feature 48 teams, up from 32 sides at the recent tournament in Qatar.
It means that there will be 104 matches played, a huge increase on the 64 games at last year's tournament.
FIFA's original plan for 2026, when the World Cup will be held in the United States, Canada and Mexico, was for 16 groups of three teams, from which the top two would advance to a last 32.
The new decided format means the top two in each group will go through to the knockout round along with the eight best third-placed sides.
As a result, the finalists, and the teams finishing third and fourth, will play a total of eight games instead of the current seven.
The decision comes after a dramatic group stage at the tournament in Qatar convinced FIFA that a rethink was needed.
"The groups of four have been absolutely incredible until the last minute of the last match," FIFA president Gianni Infantino said in December.
Tuesday's decision was announced following a FIFA Council meeting in the Rwandan capital Kigali, at which it confirmed the next men's World Cup final will be played on Sunday, July 19, 2026.
Infantino is expected to be waved in for a new four-year term as president as he stands unopposed for re-election at Thursday's FIFA Congress.
FIFA Sets 2026 World Cup Structure
KIGALI - The expanded 2026 men's World Cup in North America will start with 12 groups of four teams, a change from the original planned format of 16 groups of three, football's world governing body FIFA announced on Tuesday.
