The Women's Rugby World Cup will be increased from 12 teams to 16 from the 2025 edition.
The decision was taken by the Rugby World Cup board earlier this year and World Rugby chairman Bill Beaumont says it was a "milestone moment" for the women's game.
"Expansion of the Rugby World Cup opens additional... playing pathway opportunities for unions at the highest level of the game, and creates added incentive for unions worldwide to continue to invest and grow in their women's programmes."
The 2021 World Cup - the ninth edition since the inaugural tournament in 1991 - will be staged in New Zealand from September 18-October 16.
New Zealand are the defending champions, after winning their fifth title in 2017.
The host selection process for the 2025 edition begins in February and will conclude in May 2022 with a council vote.