Nerio I Acciaioli or Acciajuoli (full name Rainerio; died 25 September 1394) served as the de facto Duke of Athens from 1385 to 1388 and ruled uncontested until his death in 1394. Born into a family of Florentine bankers, he became the principal agent of his influential kinsman Niccolò Acciaioli in Frankish Greece in 1360. He acquired extensive estates in the Principality of Achaea, which he administered independently of the absent princes. In 1374 or 1375, he hired mercenaries and captured Megara, a strategically significant fortress within the Catalan-ruled Duchy of Athens. His forces invaded the duchy again in 1385, driving back the Catalans to the Acropolis, which they were ultimately compelled to surrender in 1388.
Nerio and his son-in-law, Theodore I Palaiologos, Despot of the Morea, invaded the Lordship of Argos and Nauplia in the Peloponnese. Nerio seized Nauplia, but the Venetians soon expelled his troops from the town. In 1389, he was captured by the Navarrese mercenary commander Pedro de San Superano, and released only after pledging to support the Venetian effort to take Argos from Theodore. As security for this promise, he ceded parts of his domains to Venice, though he failed to persuade his son-in-law to surrender Argos. In 1390, Nerio's forces captured the Duchy of Neopatras from the Catalans, but the Ottoman sultan Bayezid I conquered the territory in 1393, after which Nerio paid him an annual tribute for Athens. On 11Â January 1394, King Ladislaus of Naples, who claimed suzerainty over Frankish Greece, invested Nerio with the Duchy of Athens. In his last will, Nerio divided his possessions among his younger daughter, Francesca, his illegitimate son, Antonio, and the Church of Saint Mary (the Parthenon) of Athens.