Mojtaba Khamenei will replace his father, Ayatollah Ali Khamenei, as Iran's new supreme leader.
A statement by the Assembly of Experts - an 88-member clerical body tasked with selecting the supreme leader - was read aloud by a presenter on state TV.
Mojtaba Khamenei takes over from his father - who was killed in the first wave of US and Israeli strikes last week - as the country's spiritual leader and highest authority.
Many expect Mojtaba Khamenei to continue his father's hardline policies.
The presenter on state TV announced the decision, saying that it came "despite the acute wartime conditions and the direct threats of the enemies against this popular institution".
