This paper assesses the reality and sources of the Constitutionnel's sea serpent case, attributing to this newspaper the authorship of the invention of this journalist's joke (hoax), circulated as a running gag during the 19th century. The subject is not so much the dissemination of sea serpent stories as this problematic attribution, which became traditional in the history of the French press. At the end of our inquiry, after what might seem like a police investigation, it is established that the first mention of a sea serpent indeed really took place in this newspaper in 1817-18, when it was called the Journal du commerce, a paper that had no exclusivity in the dissemination of this hoax, which had its source in American papers. Before the digitization of newspapers, it was difficult if not impossible to find brief mentions or even articles on these elusive subjects. It was therefore easy to make fun of them without having to provide any proof or justification.