Iranian security officials have defused a handmade bomb on board a domestic flight headed from Ahvaz, in the west of Iran, to the capital Tehran, Fars news agency reported on Sunday.
"Last night, 15 minutes after the plane with 131 passengers took off, flight security guards found a handmade bomb placed in the lavatory," Fars said. "The plane landed immediately in Ahvaz airport and the bomb was defused."
It said the Kish Air flight had taken off at 10:20 pm (1750 GMT) on Saturday.
Ahvaz is the capital of Iran's oil-rich Khuzestan province, which borders Iraq and the Gulf. Sunday's incident occurred as Iran prepares for presidential elections on June 12.
On Thursday a suicide bomber killed 25 and wounded 125 worshippers in a Shiite mosque in Zahedan in southeastern Sistan-Baluchestan province bordering Afghanistan and Pakistan.
The province, which is home to Iran's Baluch minority who adhere to Sunni Islam, has been the scene of a deadly insurgency by a Sunni rebel group which is strongly opposed to the government of predominantly Shiite Iran.
The attack on the mosque was reminiscent of a similar outbreak of violence just days before Iran's last presidential election in 2005 which brought Mahmoud Ahmadinejad to power.
Bombs hit Tehran and Ahvaz, which has a sizable Arab minority, in June 2005, killing at least eight people and wounding scores more.
Some 500 kilometres (320 miles) southeast of Tehran and 50 kilometres from the Iraqi border, Ahvaz was also rocked by ethnic violence in April 2005.
Iran in the past has blamed US and British agents in neighbouring Iraq and Afghanistan for launching attacks on border provinces with significant ethnic minority populations.