Jarba, president of the Syrian National Coalition, has sought to overthrow President Bashar al-Assad for the past three years. Earlier this month Jarba visited Washington and shared a podium with US Secretary of State John Kerry.
Jarba is not running against Assad in a June 3 presidential vote but Iran's Deputy Foreign Minister Hossein Amir Abdollahian hit out regardless.
"In our opinion, when it comes to politics, Ahmad Jarba is a weak person ... He is the most inappropriate person for the future of Syria," the official IRNA news agency quoted Amir Abdollahian as saying.
The leftwing PMOI, took up arms against Iran's rulers after the 1979 Islamic revolution. Tehran holds the organisation responsible for murdering thousands of Iranian civilians and officials.
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Shiite Iran is a staunch supporter of Assad's regime, which has been battling mostly Sunni rebels who have been trying to oust him.
Amir Abdollahian, meanwhile, said a parliamentary delegation would travel to Syria to monitor the vote count.
The election has been dismissed as a "farce" by the Syrian opposition and a "parody of democracy" by the United States.
During Jarba's recent US visit Kerry said the opposition chief "understands better than anybody, the stakes and the struggle and the fight against extremism," in Syria.
Syrian opposition groups and Western countries have accused Tehran of supplying Damascus with military and financial backing since the uprising against Assad broke out in 2011.
Iran says it has only sent military experts to Syria and that it has provided Assad's regime with humanitarian assistance.