Larijani, a brother of parliament speaker Ali Larijani and a member of Iran's hardline constitutional watchdog the Guardian Council, replaces Ayatollah Mahmoud Hashemi-Shahroudi whose 10-year term has ended.
Supreme Leader Ayatollah Ali Khamenei said in a letter to Larijani that he hoped the judiciary would flourish under him. Larijani, a cleric born in Iraq's holy Shi'ite city of Najaf, will serve a five-year term.
Some reformist websites had recently reported that Larijani was hesitant to accept the position because of the mass arrests of moderate detainees over unrest that erupted after the country's disputed June 12 presidential election.
Iran arrested hundreds of people after the vote during the country's worst street unrest since its 1979 Islamic revolution.
After police and security forces quelled the street turmoil, the leadership put on trial more than 100 moderates, despite the damage it might inflict on the government's legitimacy and relations with the West.
At least 200 people remain in jail, including senior moderate politicians, activists, lawyers and journalists.
The presidential poll has also plunged the country into its biggest internal crisis in the past three decades and has exposed deepening divisions in its ruling elite.
Moderate defeated presidential candidates Mirhossein Mousavi and Mehdi Karoubi say the vote was rigged to secure the re-election of President Mahmoud Ahmadinejad but the authorities have said it was the healthiest election the country has had.
