The student news agency ISNA said Ali Bagheri, a senior Foreign Ministry official, had taken over from Javad Vaeedi, who served as the deputy in charge of international affairs for the policy body, the Supreme National Security Council.
One analyst said Bagheri was a loyalist to revolutionary values, similar to Jalili. A Western diplomat said the move could indicate an effort by Jalili to increase his influence in the council which groups a range of viewpoints.
Iran is embroiled in a standoff over its nuclear ambitions with the West, which says Tehran wants to make atomic bombs. Iran says it has only peaceful plans to produce electricity.
Ultimately, nuclear policy is decided by the Supreme Leader Ayatollah Ali Khamenei, Iran's top authority who analysts say tends to reach decisions through consultation and consensus.
Bagheri took over the post last week following a decree by Jalili, ISNA said.
Jalili, who is close to Iran's hardline President Mahmoud Ahmadinejad, replaced Ali Larijani, a political rival to the president. Larijani is a conservative but is seen by the West as more open to negotiating a solution than his successor.
The diplomat said Vaeedi, a long-standing member of the nuclear team, might still play a role in helping determine nuclear decisions even without an official position on the team.
Tehran's failure to convince world powers of its peaceful intentions has led to three rounds of limited U.N. sanctions.
World powers are awaiting Iran's formal response to a package of trade and other incentives on offer if Iran reins in it nuclear work. So far, Iran has refused any such restriction.
(Reporting by Hashem Kalantari, writing by Edmund Blair; editing by Richard Balmforth)
