The former archbishop Józef Wesołowski reportedly died at 5am inside the Vatican today.
The Vatican had placed the Polish laicised papal ambassador, 67, under house arrest as he awaited criminal trial for sexually abusing young boys.
Wesołowski served as nuncio to the Dominican Republic until August 2013.
The Vatican announced in June that a canonical court had investigated him on charges of sex abuse in the Dominican Republic.
It resulted in Wesołowski being dismissed from the clerical state, depriving him of all the rights and duties associated with being a priest except the obligation of celibacy.
The Vatican announced he would face a criminal trial, and placed him under house arrest due to his health.