Buffett, chairperson of Berkshire Hathaway Inclusion and a hugely influential investor, was with his wife when she died, the company said.
Susan Buffett, whose age was not immediately available, was a director at Berkshire and stood to inherit her husband's fortune.
Buffett has said his stock in Berkshire, valued at about $42.9bn, would go to his wife upon his death and then to a foundation.
Buffett and his wife married in 1952 and have lived separately for many years. While she spent much of her time in San Francisco, they often traveled and spent time together.
The Buffetts had three children, Susie, 50, who lives in Omaha, Howard, 49, who lives near Decatur, Illinois, and who also is on Berkshire's board, and Peter, 46, who is a musician living in Milwaukee.