The Seine creates a particular challenge for authors of urban mysteries attempting to unveil the secrets of Paris: it would seem just as incongruous to omit this unavoidable component of the city’s geography as it would be to claim this well-known river as a mystery. Thus, several of the novels chosen embrace the second option, while also painting a complex picture of the Seine as a space organized around a cast of characters engaged in odd jobs, which are mostly unknown to the reader. Two figures emerge: the chiffonnier, or the rag picker, and the pirate, both of which are used to provide a socioeconomic representation of the river. By examining the terms of this representation, this paper aims to show how urban mysteries place the expanse of the Seine within the big city.