President Gianni Infantino's favoured plan, which will see the finals comprising 16 groups of three teams before a 32-team knockout stage, was voted for by the FIFA Council at a meeting in Zurich on Tuesday night (AEDT).
It meets Infantino's election pledge of a bigger World Cup, and should help fund promised raises for FIFA's 211 member federations.
With 80 matches instead of 64, FIFA forecasts the equivalent of $1 billion extra income, at current rates, from broadcasting and sponsor deals, plus ticket sales, compared to $5.5 billion revenue forecast for the 2018 World Cup in Russia.
FIFA's six continents should find out by May how many extra places they will get.
UEFA wants 16 European teams at the tournament, which is strongly favoured to be played in North America.
The CONCACAF region has not hosted the World Cup since the 1994 tournament in the United States.
American, Canadian and Mexican football leaders have had initial talks about a co-hosting bid.
Africa and Asia could be winners in a bigger World Cup with up to nine places each - they had only five and four teams, respectively, at the 2014 World Cup in Brazil.
Still, FIFA said it expects the standard of football to drop compared to the 32-team format locked in for the next two World Cups in Russia and Qatar.
The "absolute quality" of play, defined by high-ranked teams facing each other most often, is achieved by 32 teams, FIFA acknowledged in a research document sent to members last month. It made 10,000 tournament simulations to reach that conclusion.
FIFA must break with football tradition to make their new format work after an original 48-team plan - with an opening playoff round sending 16 "one-and-done" teams home early - was unpopular.
Instead, three-team groups will replace the usual groups of four to create simple progress to a knockout bracket.
However, it leaves one team idle for final group games and could risk collusion between the other two teams.
FIFA said it could guard against result-rigging by introducing penalty shootouts after group games that end in draws.
Despite the 16 extra games, FIFA believes the current maximum of stadiums needed will stay at the 12 used by Brazil and Russia.
However, the demand for more training bases and hotels means developed countries would be better equipped to win future hosting contests.
North America is the strong favorite for 2026 because European and Asian countries are blocked by a FIFA rule excluding continents which hosted either of the two previous tournaments. Russia will host the World Cup next year and Qatar in 2022.
South America has been focused on a centenary tournament including original 1930 host Uruguay, and African nations are seen as lacking existing capacity and unlikely to fund multi-billion dollar infrastructure spending.