He also vowed that Italy would not be "intimidated" after Saturday's attack claimed by the Islamic State group that killed one civilian and injured nine more people.
"I think al-Sisi is a great leader," Renzi said in an interview with Al-Jazeera television broadcast Sunday.
"Let me be very frank. In this moment Egypt will be saved only with the leadership of al-Sisi," Renzi said in English.
"This is my personal position and I am proud of my friendship with him and I will support him in the direction of peace because the Mediterranean without Egypt will be absolutely a place without peace."
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The attack was the first on a foreign mission in Egypt since jihadists began a campaign against the country's security forces two years ago following a crackdown on Islamists.
Asked by La Repubblica newspaper about possible reasons Italy was targeted, Renzi said he would not play a "guessing game".
"Attacks like this can have random aspects, to show the logistical ability to hit a Western nation," he told the paper.
"One thing is for sure, Italy must react with quiet steadfastness. We will not allow ourselves to be intimidated."
Renzi also told Al-Jazeera it was "absolutely important" for Egypt to ensure a free media and to enable journalists to work in safety.