Warren Buffett is stepping down as a trustee of the Bill & Melinda Gates Foundation less than two months after the couple said they planned to divorce, as the multibillion-dollar charity looks to reshape its leadership structure.

People familiar with the matter said Buffett’s decision to step down is tied to the foundation’s plans to strengthen its governance and add more independence. Messrs. Buffett and Gates are longtime friends. Together with French Gates, they were the foundation’s three trustees. Unlike most large charitable groups, the foundation doesn’t have a board of directors or outside trustees.