President Ebrahim Raeisi thanked Shamkhani for his efforts during his tenure.
Ahmadian, 62, is a graduate of the University of Tehran in dentistry and has a Ph.D. in strategic management from the Supreme National Defense University of Iran.
He was the director of the Islamic Revolution Guards Corps’ Strategic Center and a member of the Expediency Council.
Ahmadian has also previously served as commander of the Islamic Revolution Guards Corps’ naval forces and the head of the IRGC joint staff.
His predecessor, Shamkhani was appointed as the country’s security chief in September 2013 by former president Hassan Rouhani.
He left his post after successfully completing a rapprochement with Saudi Arabia, which saw the two countries agree to restore diplomatic ties after a seven-year rift in a China-brokered deal in March.
Shamkhani was the second longest-serving security chief since the country’s 1979 Islamic Revolution, behind only Hassan Rouhani who was SNSC secretary for nearly 16 years.
Shamkhani was the fourth security chief of the Islamic Republic after Rouhani, Ali Larijani, and Saeed Jalili, who held the post for approximately six years.
Born in the southwestern province of Khuzestan in 1955, Shamkhani joined the IRGC in 1979. He also served as defense minister under former president Mohammad Khatami from 1997 to 2005.