With four more groups than in the last tournament (held in Qatar last year), the format change will mean that the first- and second-placed teams in each group plus the eight best third-placed sides will advance to the knockout stages.
There will thus be 32 teams in the second round of the next World Cup (to be held in the United States, Mexico and Canada), adding an additional knockout round on the route to the final.
The change, therefore, means that the four semi-finalist teams will play eight matches in total, increasing the previous maximum of seven games that has been in place since 1974.
The decision was made by the FIFA Council, meeting in Kigali, the capital of Rwanda.
"The FIFA Council unanimously approved the proposed amendment to the FIFA World Cup 2026 competition format from 16 groups of three to 12 groups of four with the top two and eight best third-placed teams progressing to a round of 32," FIFA said.
"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."
The original plan for the 2026 edition had a total of 80 matches but the decision to increase the number of games to 104 was approved by FIFA's council at the meeting on Tuesday.
The new format will also stick to drawing four teams in a group after a proposal for a three-team group was shot down over fears of collusion.
The body will hold a congress in the African nation from Thursday with FIFA President Gianni Infantino (52) expected to be re-elected for another term until 2027.