After decades of sitting on the sidelines of politics, Bill Gates, one of the richest people in the world, has said privately that he recently donated about $50 million to a nonprofit organization that is supporting Vice President Kamala Harris’s presidential run, according to three people briefed on the matter.

The donation was meant to stay under wraps. Mr. Gates, one of the founders of Microsoft, has not publicly endorsed Ms. Harris, and his donation would represent a significant change in the strategy that has previously kept him away from gifts like this.

In private calls this year to friends and others, Mr. Gates has expressed concern about what a second Donald Trump presidency would look like, according to a person briefed on Mr. Gates’s thinking, although he has stressed that he could work with either candidate. Mr. Gates does not have a deep relationship with Ms. Harris, but he has celebrated the Biden-Harris administration’s work on climate change. Mr. Gates’s philanthropic organization, the Bill & Melinda Gates Foundation, is significantly concerned about potential cuts to family planning and global health programs if Mr. Trump is elected, according to two people close to the foundation.

Mr. Gates has said he made his donation to Future Forward, the main outside fund-raising group supporting Ms. Harris, according to the people briefed on the matter. Mr. Gates has talked about his pro-Harris donation with his peers, including Mike Bloomberg, the former New York City mayor and a major supporter of Future Forward who has considered a similarly sized gift, two of the people briefed said. Mr. Gates’s donation went specifically to Future Forward’s nonprofit arm, Future Forward USA Action, which as a 501(c)(4) “dark money” organization does not disclose its donors, according to the people briefed. So any contribution by Mr. Gates will never appear on any public filing.