The agency also said that a caretaker interior minister had been appointed.
"The president has appointed Mahdi Hashemi as caretaker interior minister," IRNA announced.
Hashemi will remain in his post until a candidate nominated by Ahmadinejad has been approved by the Iranian parliament.
Although no official reason has been given for the interior minister's dismissal, there had been frequent rumors in the Iranian media in recent weeks that Pourmohammadi was about to be sacked. The rumors said that Ahmadinejad had decided to dismiss Pourmohammadi because he reported to Iranian Supreme Leader Ayatollah Ali Khamenei without the president's approval or knowledge.
This is not the first dismissal of a top minister this year in Iran. In late April, the economics minister, Davood Danesh Jafari, was dismissed over apparent policy differences with the president.
In a statement after his dismissal, Jafari accused Ahmadinejad of not listening to advice on how to tackle inflation in the Islamic Republic, which reached an official of 18.4% last year, with unofficial sources putting it at more than 20%.