Velayati, 67, is the top adviser to Khamenei on international affairs. He served as foreign minister during the 1980-88 war with Iraq and into the 1990s.
Velayati has the support of many influential figures in the conservative camp. He gave up plans to contest the 2005 presidential vote after former President Akbar Hashemi Rafsanjani announced plans to run.
After studying pediatrics in Iran, Velayati specialized in infectious diseases at Baltimore's John Hopkins University in the 1970s.
He is a physicist and runs a hospital in northern Tehran.
He has been accused by Argentina
of involvement in the 1994 bombing of a Jewish center in Buenos Aires that killed 85 people.