Nerio I Acciaioli (full name Rainerio) was as Italian aristocrat from Florence who rose to power in Frankish Greece during the last decades of the fourteenth century, eventually becoming Duke of Athens.
Nerio was the son of Jacopo Acciaioli and Bartolommea Ricasoli, and younger brother of Donato and elder brother of Giovanni. When his relative Niccolò Acciaioli, grand seneschal of Naples, who owned lands and castles in Achaea and Corinth and had created Donato his vicar in Greece, died (1371), his son and successor, Angelo Acciaioli, replaced Donato with Nerio in Greece. He participated in the crusader Council of Thebes in October 1373, but all its planning came to naught. In 1374, when the Catalan vicar general of Athens, Matthew of Peralta, died, Nerio swooped down on Megara and took it. It was the first action of his long career of conquest and aggrandisement. Subsequent to his capture of Megara, Nerio was involved in almost constant warfare with the Catalans who ruled in Athens.
In 1378, Nerio was enlisted along with the Navarrese Company by the Hospitaller Grand Master Juan Fernández de Heredia for his war with Arta in the Despotate of Epirus. Nerio, in turn, enlisted the Navarrese of Juan de Urtubia, who left the rest of the Company with about a hundred soldiers and crossed the Corinthian Gulf. In 1379, Juan de Urtubia captured Thebes.
On 7 July 1385, Nerio took up the title dominus Choranti et Ducaminis: "lord of Corinth and the Duchy of Athens." In the winter that year, he successfully fought the Ottomans. In 1386, he had annexed the lower city of Athens. He acquired the Acropolis of Athens by conquest on 2 May 1388, though a plague forced him to return with his family to Thebes shortly after.
On 29 December 1391, Nerio signed a treaty with Amadeo, Prince of Achaea, against the Navarrese. Nerio was created Duke of Athens by Ladislaus of Naples on 11 January 1394. He held this title for nine months before his death.