Sagawa served time in a French jail for the murder of the Dutch student Renée Hartevelt, a classmate at the Sorbonne Academy. On June 11, 1981, Sagawa, a 32-year-old student of Comparative literature, invited Hartevelt to dinner at his 10 Rue Erlanger apartment under the pretense of translating German poetry for a class he was taking. Upon her arrival, after convincing her to begin reading the poetry, he then shot her in the neck with a rifle while she sat with her back to him at a desk. At that point he began to carry out his plan of eating her. His first attempt to bite into her buttocks met with failure so he proceeded to go out shopping for a butcher knife. She was selected, he has stated, for her health and beauty, those characteristics Sagawa believed he lacked. Sagawa describes himself as a "weak, ugly, and inadequate little man" (he is just under 5 ft (1.52 m) tall) and claims that he wanted to "absorb her energy".

Sagawa said he fainted after the shock of shooting her, but awoke with the realization that he had to carry out his desire to eat her. He did so, beginning with her buttocks and thighs, after having sex with the corpse. In interviews, he noted his surprise at the "corn-colored" nature of human fat. For two days, Sagawa ate various parts of her body. He described the meat as tasting like raw tuna. He then attempted to dump the mutilated body in a remote lake, but was seen in the act and later arrested by the French police who found parts of the deceased still in his refrigerator.

His wealthy father provided a top lawyer for his defense, and after being held for two years without trial the French judge Jean-Louis Bruguière found him legally insane and unfit to stand trial and ordered Sagawa to be held indefinitely in a mental institution. Following a visit by the author Inuhiko Yomota, Sagawa's account of the murder was published in Japan with the title In the Fog. The subsequent publicity and macabre celebrity of Sagawa likely contributed to the French authorities' decision to have him extradited to Japan. Upon arrival in Japan, he was immediately taken to Matsuzawa hospital, where examining psychologists all found him to be sane, stating that sexual perversion was the sole motivation for the murder. However, Japanese authorities found it to be legally impossible to hold him, because the French government refused to release court documents (which remain secret), to Japan, claiming that the case was already dropped in France. As a result, Sagawa checked himself out of the mental institution on August 12, 1986, and has been a free man ever since. Sagawa's freedom has been questioned and criticized by many.