The decision on Sunday represents a stark reversal for the billionaire investor, who for years had said his wealth would be pledged to philanthropies after his death.
Moreover, his vast holdings of Berkshire Hathaway Inc stock had been expected to go largely to the Susan Thompson Buffett Foundation, begun by his late wife.
Her foundation has given millions of dollars to hospitals, universities and teachers, as well as to Planned Parenthood and other abortion rights groups.
Instead the 75-year-old chairman and chief executive of Berkshire Hathaway plans to give the largest contribution to the Bill and Melinda Gates Foundation, established by Buffett's friend and Berkshire board member Bill Gates.
Buffett's gift will increase the resources of the Gates Foundation, which is already the world's largest philanthropy with assets of about $29.1 billion.
Calls to Berkshire Hathaway's offices were not immediately returned on Sunday, but letters outlining the gifts were posted on the company's web site. Fortune magazine reported on Buffett's decision earlier.
The Gates' issued a joint statement applauding Buffett's decision.
"We are awed by our friend Warren Buffett's decision to use his fortune to address the world's most challenging inequities, and we are humbled that he has chosen to direct a large portion of it to the Bill & Melinda Gates Foundation," Bill and Melinda Gates said.
Over time Buffett plans to give away 12,050,000 Class B shares of Berkshire Hathaway stock to the foundations, but he will have to convert some of his 498,320 Class A shares to complete the gifts.
The gifts would be worth nearly $37 billion at Friday's Class A share prices.
"We are awed by our friend Warren Buffett's decision to use his fortune to address the world's most challenging inequities"
Bill and Melinda Gates
Buffett said he plans to earmark 10 million B shares for the Gates Foundation, 1 million B shares for the Susan Thompson Buffett Foundation named in honour of his wife and 350,000 shares for the three foundations run by each of his children.
Buffett plans to give each foundation 5% of his total pledge each year in July.
Buffett's health has been the subject of speculation. He has said a succession plan is in place at Berkshire but refuses to name a successor.