Melinda French Gates will step down as co-chair of the Bill & Melinda Gates Foundation, the nonprofit she and her ex-husband Bill Gates founded and built into one of the world's largest philanthropic organisations over the past 20 years.

"This is not a decision I came to lightly," Ms French Gates posted on the X platform on Monday.

She praised the foundation's CEO, Mark Suzman, and the foundation's board of trustees, which was significantly expanded after the couple announced their divorce in May 2021.

"The time is right for me to move forward into the next chapter of my philanthropy," Ms French Gates wrote in her statement.

She organises some of her investments and philanthropic gifts through her organisation, Pivotal Ventures, which is not a nonprofit.

Bill Gates thanked Ms French Gates for her "critical" contributions to the foundation in a statement, saying, "I am sorry to see her leave, but I am sure she will have a huge impact in her future philanthropic work."

The foundation will change its name to the Gates Foundation, a spokesperson said.

Ms French Gates will receive $US12.5 billion ($18.9 billion) as part of her agreement with Mr Gates, which she said she would commit to future work focused on women and families.

The foundation said that Mr Gates would supply those funds and they would not come out of the foundation's endowment.

The Gates Foundation is a massive funder of global health, supporting major international institutions like Gavi, the Vaccine Alliance, the World Health Organization and The Global Fund to Fight AIDS, Tuberculosis and Malaria.

It also funds research into a wide range of topics like child malnutrition and maternal health as well as eradicating polio and treating and preventing malaria.

The foundation has also funded with billions of dollars the development of interventions and research meant to help small farmers adapt to climate change.

In the US, it funded education policy and research that had sweeping, if mixed, effects, and now, has pledged to increase its support around anti poverty initiatives.

In an emailed statement, the foundation said that Mr Suzman announced Ms French Gates' decision to employees on Monday.

"After a difficult few years watching women's rights rolled back in the US and around the world, she wants to use this next chapter to focus specifically on altering that trajectory," Mr Suzman said of Ms French Gates.

Mr Suzman said he knew many had joined the foundation in part because of their admiration for her advocacy, especially around gender equity.

The Gates Foundation holds $US75.2 billion in its endowment as of December 2023, and announced in January, it planned to spend $US8.6 billion through the course of its work in 2024.

The Associated Press receives financial support for news coverage in Africa from the Bill & Melinda Gates Foundation and for news coverage of women in the workforce from Pivotal Ventures.

AP