Nerio I Acciaioli or Acciajuoli (full name Rainerio; died 25 September 1394) was the de facto Duke of Athens from 1385 to 1388, after which he reigned uncontested until his death in 1394.

Born to a family of Florentine bankers, he became the principal agent of his influential kinsman, Niccolò Acciaioli, in Frankish Greece in 1360.

King Ladislaus of Naples, who claimed suzerainty over Frankish Greece, invested Nerio with the Duchy of Athens on 11 January 1394.

In his last will, Nerio distributed his domains between his younger daughter, Francesca, his illegitimate son, Antonio, and the church of Saint Mary (the Parthenon) of Athens.

[1] The Acciaioli were a Florentine banking family who regularly lent money to the Angevin monarchs of Naples in the 14th century.

[2] Niccolò seized large estates in Achaea during the following decades, but his children showed little interest in the Greek affairs.

[5] Niccolò Acciaioli persuaded Pope Innocent VI to appoint Nerio's younger brother, John, to the important Archbishopric of Patras in Achaea in May 1360.

[5] Next year, Niccolò and John Acciaioli decided to arrange Nerio's marriage with Florence Sanudo, Duchess of the Archipelago (or Naxos).

[5] Flerence's suzerains, Queen Joan I of Naples and Robert of Taranto supported their plan and forbade Florence to marry without their consent, but the Venetians abducted her to prevent the marriage.

[4] Robert of Taranto's wife, Marie of Bourbon, sold two Achaean baronies, Vostitza and Nivelet, to Nerio for 6,000 ducats in 1363 or 1364.

[12] He captured Megara with the support of its burghers and imprisoned its Catalan commander, Francis Lunel, in late 1374 or early 1375.

[16] Nerio made contact with one of the Navarrese commanders, Juan de Urtubia, and persuaded him to invade the Duchy of Athens in early 1379.

[14][18] The Catalans of Athens held a general assembly and acknowledged King Peter IV of Aragon as their lawful ruler on 20 May 1380.

[22] San Superano and Varvassa returned to Achaea and swore fealty to Philip of Taranto's nephew, James of Baux, who had laid claim to the principality.

[21] The Navarrese started expanding their rule towards Corinth, but Nerio concluded an alliance with Theodore I Palaiologos, Despot of the Morea, against them.

[23] Nerio also entered into negotiations with the Venetian officials of Euboea to organize joint military actions against the Turks who were making raids against the Greek coasts.

[25][26] They occupied the lower town of Athens, but could not capture the Acropolis from Dalmau's deputy, Raymond de Vilanova.

[26] Nerio styled himself as "lord of the castellany of Corinth, the duchy of Athens and their dependencies" in a letter of grant on 15 January 1387.

[25][30] Nerio dispatched his troops to continue the conquest of Catalan territories, but his alliance with Theodore I Palaiologos brought him into conflict with Venice.

[32] The Venetians made an alliance with Pedro de San Superan and Paolo Foscari, Archbishop of Patras, and sent commissioners to take possession of both towns.

[30] San Superano invited Nerio to Vostitza (which had become an important fortress of the Navarrese Company) to start negotiations about Argos with him.

[36] Donato proposed to cede Athens, Thebes and movables as pledge to Venetians for Nerio's support for the surrender of Argos to them.

[47] The elder of the two daughters of Nerio by Agnes, Bartolomea, was famed for her beauty, according to the Byzantine historian, Laonikos Chalkokondyles.