French psychologist and neurologist influential in bringing about in France and the United States a connection between academic psychology and the clinical treatment of mental illnesses. He stressed psychological factors in hypnosis and contributed to the modern concept of mental and emotional disorders involving anxiety, phobias, and other abnormal behaviour.

Janet's report (1882) of an unusual case of hypnosis and clairvoyance gained him the attention of neurologist Jean-Martin Charcot (Charcot, Jean-Martin). As a Ph.D. candidate at the University of Paris, Janet studied automatic acts, and in his thesis (1889), which went into many editions, he introduced but did not amplify the concept of the unconscious. This work engendered a later dispute with Sigmund Freud (Freud, Sigmund) over priority. At Charcot's invitation, Janet became director of the psychological laboratory at the largest Paris mental institution, the Salpêtrière Hospital (1889). There he completed his work for his M.D., which he received for the thesis L'État mental des hystériques (1892; The Mental State of Hystericals, 1901), in which he attempted to classify forms of hysteria. Charcot, in his introduction to the thesis, concurred with Janet's plea to unite the efforts of clinical and academic psychology.

Elected to a chair in psychology at the Sorbonne (1898), Janet succeeded to Théodule-Armand Ribot (Ribot, Théodule-Armand)'s chair in the Collège de France (1902–36). He was also a practicing physician specializing in nervous and mental disorders. He wrote and lectured on a wide range of subjects, including impairment of the ability to act independently, hysteria, obsession, amnesia, and personality.