Iran's Supreme Leader Ayatollah Ali Khamenei has unexpectedly replaced the commander of the elite Islamic Revolution Guards Corps.
Gen Jafari commanded the guards ground forces for 13 years
The new chief, Gen Mohammad Ali Jafari, commanded the Revolutionary Guards ground forces for 13 years.
His predecessor, Gen Yahya Rahim Safavi, has been appointed a special military adviser to Ayatollah Khamenei.
The decision comes just two weeks after US officials said they might designate the guards a "terrorist" organisation.
Washington has accused the force of helping to destabilise Iraq and Afghanistan by supplying and training insurgents.
The Revolutionary Guards condemned the plan as "worthless resolutions" issued "under baseless pretexts... to damage this holy institution".
In a decree issued on Saturday, Ayatollah Khamenei pointed to Gen Jafari's "brilliant background in the armed forces in different stages", state-run media reported.
The 50-year-old reportedly won both popularity and respect among the guards for his role as a commander during the 1980-1988 war with Iraq.
Officially the Islamic Revolution Guards Corps (IRGC), or Pasdaran
Formed after 1979 revolution
Loyal to clerics and counter to regular military
Estimated 125,000 troops
Includes ground forces, navy, air force, intelligence and special forces
Also has political influence: dozens of ex-guard sit as MPs
Iran President Ahmadinejad is a former member
No reason was given for the surprise move, but Iranian officials insisted the change was routine and that Gen Safavi's appointment should be seen as a promotion.
"Appointments made by His Eminence are usually for no longer than 10 years," Gen Safavi told the Iranian television channel, IRTV1. "Such movements are natural."
The Revolutionary Guards force was set up shortly after the 1979 Iranian revolution to defend the country's Islamic system, and to provide a counterweight to the regular armed forces.
It has since become the dominant military force in Iran, with past members including President Mahmoud Ahmadinejad and a number of his cabinet ministers.
It is estimated to have 125,000 active members, and boasts its own ground forces, navy and air force. It also controls the paramilitary Basij Resistance Force and the powerful bonyads, or charitable foundations, which run a considerable part of the Iranian economy.
The US has accused the Revolutionary Guards overseas operation arm, the Quds Force, of supplying powerful roadside bombs to Shia militants in Iraq.