Plato is often mentioned in Manzoni’s writings, yet the link between the Greek philosopher and the Italian novelist has not been so far adequately discussed. The present article aims to shed light on this topic, with specific regard to Manzoni’s evaluation of Plato’s style. Most notably, the paper will analyse some «idiotismi» from the Theaetetus, thereby highlighting the attraction of the Italian novelist for the popular expressions used by the philosopher.