Tractatus coislinianus is an ancient Greek manuscript outlining a theory of comedy in the tradition of Aristotle's Poetics.
The Tractatus states that comedy invokes laughter and pleasure, thus purging those emotions (catharsis), in a manner parallel to the description of tragedy in the Poetics.
The manuscript now resides in Paris, France, at the Bibliothèque Nationale; it is labeled "Coislinianus 120".
A. Cramer, poring through the collection of Henri Charles du Cambout de Coislin, was struck by the content, believing it to be the work of a commentator on Aristotle's theory of comedy, and published it in 1839.
This belief was soon derided, but it has gained force in the 20th century; Richard Janko has argued that it is the notes or sketches of the lost second section of the Poetics.