According to photos taken by tourists shared on Italian media, the man had an inscription painted on his back calling for the children of Ukraine to be saved.
The Vatican said the man also had self-inflicted cuts on his body from his fingernails.
Vatican guards handed the man, who was not identified, over to Italian police. Reuters reported.
The episode occurred just before the basilica closed on Thursday afternoon, June 1.
Russia invaded Ukraine on February 24, 2022,marking the largest war in Europe since World War II.