The cause of death was not immediately clear, but Tanumafili was said to have been being treated in hospital for pneumonia.
He had been king since the tiny nation, populated by about 180,000 over seven volcanic islands, gained independence from New Zealand in January 1962.
Although Tanumafili was made head of state for life when Samoa gained independence 45 years ago, under Samoa's constitution his successor will be appointed for only five-year terms and will be decided by a vote of the country's elected legislative assembly.