The protagonist of the novel, he is also the narrator, writing down his observations in diary format. After traveling around most of Africa and the Far East, he returned to Bouville to complete his historical research on the Marquis de Rollebon. Yet, not only has he lost interest in his research, but something about the way he sees both himself and the outside world begins to worry him. Whether it is holding a stone or looking at a glass of beer, he feels confronted by the bare existence of things. The result is what he calls the Nausea. He soon realizes that the Nausea comes from the fact that "existence precedes essence." He thinks the physical characteristics of objects and people are just a comforting facade to mask the "nothingness" of existence. By the end of the novel he has disavowed the past, embraced his existence, and discovered that there is no purpose to existence. Rather than surrender to despair, he decides to assert his freedom and moves to Paris to write a novel.
Although not a character in the novel per se, he is the subject of Roquentin's research. He was a mysterious French aristocrat who meddled in politics during and after the French Revolution. At first Roquentin thinks he can learn everything about him, but soon realizes that not only is he guessing about who the marquis really was, but he is also using him to justify his existence. Roquentin's rejection of Rollebon is thus a rejection of living in the past.
She is Roquentin's old lover and lives in Paris. Even though she begs Roquentin to come see her, she is more interested in the man he used to be. She admittedly lives in the past, rereading the same history books and recalling the "perfect moments" of her life. She refuses to resume her relationship with Roquentin since she is already the mistress of a number of men who pay for her apartment.
A lonely man whom Roquentin meets at the Bouville Library. Roquentin mocks him for thinking he can learn all there is to know by reading everything in the Library in alphabetical order. He is a defender of humanism, believing that all men and women are united by the common bond of love. He is later chased out of town for fondling a small boy in public.
The barmaid of a local cafe, she is Roquentin's sometime lover. As his nausea progresses he is more and more disgusted at the prospect of having sex with her.