"The Mediterranean is not the border but the heart of Europe," he said at a news conference with his Tunisian counterpart Mehdi Jomaa.
"I chose Tunisia for my first foreign visit as prime minister to show the centrality that Italy attaches to the Mediterranean," he said.
Renzi, who took office last month, also promised Mediterranean affairs would be central for Italy when it assumes the rotating presidency of the European Union in the second half of 2014.
He also said he discussed with Jomaa cooperation over the issue of illegal immigration, as many of the makeshift boats carrying would-be migrants to Europe from Tunisia and Libya end up on the Italian island of Lampedusa.
"We believe this cooperation can be a model," said Renzi, promising "full support," without elaborating, while claiming to have found an "incredible decrease in this sector."
The European Union and Tunisia on Monday signed an agreement which includes a visa facilitation and greater openness to legal immigration in exchange for the fight against illegal immigration.
For his part, Jomaa hailed the visit of Renzi, saying it would "come up with new ideas including... on Mediterranean and bilateral relations."