Iranian media reported that Salehi will be the interim foreign minister until the parliament officials endorses his appointment.
Under Iranian law, the president has to submit his nominations for ministerial posts to parliament for approval.
Salehi, who continues to head Iran's atomic energy body, took charge officially at a function which was also the farewell ceremony for his predecessor Manouchehr Mottaki who however was not present, media reports said.
Mottaki, 57, was fired on December 13 by Ahmadinejad during an official visit to Senegal.
His dismissal came after he hailed as a "step forward" remarks by US Secretary of State Hillary Clinton that Iran is entitled to a peaceful nuclear energy programme.
Clinton had told the BBC that Tehran could enrich uranium for civilian purposes in the future, but only once it has demonstrated it can do so in a responsible manner and in accordance with Iran's international obligations.
Mottaki's comments appeared to cut across the Islamic republic's official position, repeated almost daily, that its enrichment of uranium is non-negotiable.
Mottaki's sacking also came just days after Iran held crunch talks in Geneva on December 6 and 7 with world powers over its controversial nuclear dossier. Further talks are scheduled for next month in Iran's neighbour Turkey.
Salehi, 61, who was appointed atomic energy chief on July 17, 2009, has been a driving force behind Iran's atomic programme, and during his tenure, the country's first nuclear power plant has come on line.
Before taking up the post, he was deputy chief of the Jeddah-based Organisation of the Islamic Conference.
Salehi is a PhD graduate of the prestigious MIT in the United States.
He served as Tehran's representative in the Vienna-based International Atomic Energy Agency during the presidency of the reformist Mohammad Khatami.
TEHRAN — AFP