Paul Alexis (1847-1901) est souvent cité comme le meilleur ami de Zola, c’est là sa principale caractéristique. Presque son unique définition. On a tendance à oublier qu’il est aussi un écrivain engagé dans une cause qui est le principe même de son existence : le naturalisme. Ce travail tente d’explorer cette double perspective. En étudiant un certain nombre d’ouvrages et d’articles de journaux de cet auteur, il essaie de comprendre d’un côté ses rapports avec Zola, et de l’autre, ses conceptions personnelles, ses convictions profondes ainsi que ses relations avec les autres membres du mouvement. Pour cela, il scrute, tout d’abord, ses liens à la fois littéraires et intimes avec Zola. Il montre le quotidien d’une amitié qui connaît tumultes et mutations. Il analyse les moments marquants qui ont rythmé le cours de leur vie. Il note les convergences et tente de mettre en lumière les divergences entre le rédacteur de Germinal et celui de Madame Meuriot qui se présentait comme le tenant quasi ultime des thèses naturalistes. Ensuite, il s’efforce d’exposer précisément les idées personnelles de cet écrivain à partir de ses prises de position théoriques et de ses ouvrages de fiction. Il observe son implication dans ses domaines de prédilection : le journalisme, les arts, le théâtre, la politique... Enfin, il décrit son action menée au côté de Zola, au cœur du cercle de Médan, mais aussi d’une façon autonome, face aux grands événements qu’il a rencontrés, face aux grands problèmes qu’il a affrontés, comme l’émergence du naturalisme ou l’Affaire Dreyfus, par exemple. Le but ultime de cette thèse est de s’intéresser à ce personnage décrié souvent, parfois négligé, qui fut plus qu’un compagnon de route de Zola. / Paul Alexis (1847-1901) is mainly known as the closest friend of Zola. This is quoted as his main characteristic, almost as his unique definition. He is usually forgotten to be mentioned as also a writer who was very dedicated to a cause which had a huge significance in his life : Naturalism. This thesis tries to explore this double perspective. By studying books and newspaper articles written by this author, this work aims on the one hand, to explore his relationship with Zola, and, on the other hand, to focus on his personal designs, his deep convictions, as well as his connections with the other members of the movement. Primarily, in order to achieve this, this thesis scrutinises his personal and literary links with Zola. It shows the everyday life of a friendship which went through many challenges and changes. It analyses the striking moments which occurred during the course of their lives. It emphasises their common points of thought and tries to highlight the differences between the author of Germinal and the writer of Madam Meuriot, the last appearing to be the ultimate upholder of the naturalistic theories. Then, this project strives to precisely explain the personal ideas of this novelist, according to his theoretical views and to his works of fiction. It observes his restless involvement and the fights he led for the sake of his favourite topics such as : journalism, arts, theatre, politics... Finally, this work describes the actions Alexis carried out at the side of Zola, at the heart of the circle of Médan, but also in a more autonomous way, during major events in which he participated. This paper aims to clarify the huge issues he faced, such as the rise of naturalism or the Dreyfus affair, for example. The ultimate purpose of this PhD is to consider this character often slandered, sometimes neglected, a man who was more than a fellow traveller of Emile Zola.