Rouhani will also visit the Vatican and France on his first overseas trip since the nuclear deal came into force earlier this month, clearing the way for Iran to rebuild its relationship with the West.
The Iranian leader touched down in Rome accompanied by more than 100 ministers, officials and businessmen who are expected to help him clinch deals worth billions in trade and investment, topped by a major order for new Airbus planes.
The fight against the Islamic State group, whose attacks on Paris forced Rouhani to delay a trip originally scheduled for November, and the war in Syria are expected to feature highly in diplomatic contacts during the visit.
Rouhani, 67 and a former academic and diplomat, is seen as a pragmatist who was elected in 2013 on a pledge to end sanctions and improve relations with the West.
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He was scheduled to meet Italian Prime Minister Matteo Renzi on Monday evening and will be received by Pope Francis on Tuesday and by French President Francois Hollande on Wednesday.
He also revealed that "important contracts" were in the works with French car makers Peugeot and Renault, adding to a burgeoning list of deals being struck as European companies scramble to get back into a $400-billion (370-billion-euro) economy with the fourth biggest reserves of oil in the world and a consumer market of 80 million people.
National carrier Iran Air said on Sunday it would be buying 114 Airbus planes to modernise an ageing fleet that has struggled to stay in the air as a result of the impact of sanctions.
Iran's Transport Minister Abbas Akhoundi said the first Airbuses were earmarked for delivery by March and that Iran was in the market for a total of up to 500 planes.
Peugeot is tipped to forge a car assembly joint venture with Iran Khodro, reviving a partnership which generated Iranian sales of 473,000 units in its last year before the French company pulled out in 2012.