Trump, on a tour that has already taken him to Saudi Arabia, Israel and the Palestinian territories, touched down in the Italian capital aboard Air Force One just before 1630 GMT, an AFP staffer travelling with the presidential team said.
The under-fire billionaire is due at the Vatican on Wednesday morning for a meeting with the pope, with whom he has clashed on numerous issues ranging from the plight of migrants to unbridled capitalism and climate change.
After the papal audience, also to be attended by Trump's wife Melania and daughter Ivanka, the Trump couple are to be given a tour of the Sixtine Chapel and St Peter's Basilica.
The president will then meet Italy's president and prime minister at the presidential palace, the Quirinale, while Melania visits a children's hospital and Ivanka discusses migrant issues with members of the St Egidio religious community.
The Trump team is due to fly to Brussels on Wednesday afternoon for meetings with EU and NATO officials before returning to Italy late Thursday for the G7 summit in Sicily on Friday and Saturday.