Earlier on Monday his foreign minister, Massimo D'Alema, warned in Paris that executing Saddam would be an "unacceptable" mistake that could throw Iraq into a "veritable civil war".
Prodi, who was speaking in London following talks with his British counterpart, Tony Blair, was more general in his opposition.
"Italy is against the death penalty and so even in such a dramatic case as Saddam Hussein we still think that the death penalty must not be put into action," Prodi said.
Blair told his monthly press conference earlier on Monday that he was against the death penalty for "Saddam or anybody else" but remained otherwise tight-lipped on the sentence passed on the former Iraqi dictator.
Saddam was sentenced to hang by the Iraqi High Tribunal, which found him guilty on Sunday of crimes against humanity in the case of 148 Shia civilians who were allegedly killed in revenge for a 1982 attempt on his life.
Saddam's lawyers launched an appeal on Monday. Under Iraqi law, if it fails he must be executed within 30 days.

Copyright AFP (Agence France-Presse), 2006