Unconfirmed. Some media and social reports claim Mojtaba Khamenei (son of the late Ali Khamenei) has been named successor, but Iranian state media, BBC, Reuters, and officials make no such announcement.
Ali Khamenei's death was confirmed today after US-Israeli strikes. Per Iran's constitution, the 88-member Assembly of Experts selects the new Supreme Leader; an interim council (president, judiciary head, Guardian Council jurist) handles transition now. Mojtaba is a speculated frontrunner, nothing official yet.