The aim of this article is to investigate the use of irony in the Encyclopédie by Denis Diderot. Encyclopédie, indeed, is also a dictionary and the literary trope of irony seems too ambiguous and dual to be appropriate for definitions and descriptions. However, Diderot, who is sometimes ironic, uses irony also in this particular literary context, just like Socrates who used this rhetorical figure as a critical tool. What is argued here is the fact that he uses irony for three main purposes: to show the obscurity of some words, especially exotic ones; to demystify superstitions and religious belief; to demonstrate the right method to acquire knowledge. There are some other philosophical reasons to be ironic in the Encyclopédie, briefly considered in the article.