Iran's supreme leader Ayatollah Ali Khamenei appointed the head of the judiciary, Sadegh Amoli Larijani, as chairman of the powerful Expediency Council on Sunday, according to his website.
The Expediency Council is a key behind-the-scenes institution, settling disputes between different branches of government.
It was led for many years by one of the Islamic republic's founding figures, Akbar Hashemi Rafsanjani, until his death in early 2017.
Larijani, 58, was personally sanctioned by the United States in January 2018 for "human rights abuses" and "supporting Iranian weapons programs", according to the US Treasury.
He was also appointed to the 12-man Guardian Council -- a post he previously held -- that vets legislation and election candidates, according to a statement on Khamenei's website.
A new judiciary chief was expected to be named shortly.
Larijani takes over the Expediency and Guardian council posts from Ayatollah Hashemi Shahroudi, who died last week.
He is one of five Larijani brothers who all hold powerful positions within the establishment, most notably his eldest sibling Ali Larijani who is speaker of parliament.
Khamenei's statement said "critical changes" were expected within the Expediency Council in the near future, without giving details.