Jacques Marie Émile Lacan (UK: /læˈkɒ̃/, US: /ləˈkɑːn/ lə-KAHN; French: [ʒak maʁi emil lakɑ̃]; 13 April 1901 – 9 September 1981) was a French psychoanalyst and psychiatrist. Described as "the most controversial psycho-analyst since Freud", Lacan gave yearly seminars in Paris, from 1953 to 1981, and published papers that were later collected in the book Écrits. Transcriptions of his seminars, given between 1954 and 1976, were also published. His work made a significant impact on continental philosophy and cultural theory in areas such as post-structuralism, critical theory, feminist theory and film theory, as well as on the practice of psychoanalysis itself.

Lacan took up and discussed the whole range of Freudian concepts, emphasizing the philosophical dimension of Freud's thought and applying concepts derived from structuralism in linguistics and anthropology to its development in his own work, which he would further augment by employing formulae from predicate logic and topology. Taking this new direction, and introducing controversial innovations in clinical practice, led to expulsion for Lacan and his followers from the International Psychoanalytic Association. In consequence, Lacan went on to establish new psychoanalytic institutions to promote and develop his work, which he declared to be a "return to Freud", in opposition to prevalent trends in psychology and institutional psychoanalysis collusive of adaptation to social norms.

Lacan was born in Paris, the eldest of Émilie and Alfred Lacan's three children. His father was a successful soap and oils salesman. His mother was ardently Catholic – his younger brother entered a monastery in 1929. Lacan attended the Catholic Collège Stanislas de Paris between 1907 and 1918. An interest in philosophy led him to a preoccupation with the work of Spinoza, one outcome of which was his abandonment of religious faith for atheism. There were tensions in the family around this issue, and he regretted not persuading his brother to take a different path, but by 1924 his parents had moved to Boulogne and he was living in rooms in Montmartre.

During the early 1920s, Lacan actively engaged with the Parisian literary and artistic avant-garde. Having met James Joyce, he was present at the Parisian bookshop where the first readings of passages from Ulysses in French and English took place, shortly before it was published in 1922. He also had meetings with Charles Maurras, whom he admired as a literary stylist, and he occasionally attended meetings of Action Française (of which Maurras was a leading ideologue), of which he would later be highly critical.

In 1920, after being rejected for military service on the grounds that he was too thin, Lacan entered medical school. Between 1927 and 1931, after completing his studies at the faculty of medicine of the University of Paris, he specialised in psychiatry under the direction of Henri Claude at the Sainte-Anne Hospital, the major psychiatric hospital serving central Paris, at the Infirmary for the Insane of the Police Prefecture under Gaëtan Gatian de Clérambault and also at the Hospital Henri-Rousselle.

Lacan was involved with the Parisian surrealist movement of the 1930s, associating with André Breton, Georges Bataille, Salvador Dalí, and Pablo Picasso. For a time, he served as Picasso's personal therapist. He attended the mouvement Psyché that Maryse Choisy founded and published in the Surrealist journal Minotaure. "[Lacan's] interest in surrealism predated his interest in psychoanalysis," former Lacanian analyst and biographer Dylan Evans explains, speculating that "perhaps Lacan never really abandoned his early surrealist sympathies, its neo-Romantic view of madness as 'convulsive beauty', its celebration of irrationality." Translator and historian David Macey writes that "the importance of surrealism can hardly be over-stated... to the young Lacan... [who] also shared the surrealists' taste for scandal and provocation, and viewed provocation as an important element in psycho-analysis itself".

In 1931, after a second year at the Sainte-Anne Hospital, Lacan was awarded his Diplôme de médecin légiste (a medical examiner's qualification) and became a licensed forensic psychiatrist. The following year he was awarded his Diplôme d'État de docteur en médecine [fr] (roughly equivalent to an MD degree) for his thesis "On Paranoiac Psychosis in its Relations to the Personality" ("De la Psychose paranoïaque dans ses rapports avec la personnalité"). Its publication had little immediate impact on French psychoanalysis but it did meet with acclaim amongst Lacan's circle of surrealist writers and artists. In their only recorded instance of direct communication, Lacan sent a copy of his thesis to Sigmund Freud who acknowledged its receipt with a postcard.

Lacan's thesis was based on observations of several patients with a primary focus on one female patient whom he called Aimée. Its exhaustive reconstruction of her family history and social relations, on which he based his analysis of her paranoid state of mind, demonstrated his dissatisfaction with traditional psychiatry and the growing influence of Freud on his ideas. Also in 1932, Lacan published a translation of Freud's 1922 text "Über einige neurotische Mechanismen bei Eifersucht, Paranoia und Homosexualität" ("Some Neurotic Mechanisms in Jealousy, Paranoia and Homosexuality") as "De quelques mécanismes névrotiques dans la jalousie, la paranoïa et l'homosexualité" in the Revue française de psychanalyse [fr]. In Autumn 1932, Lacan began his training analysis with Rudolph Loewenstein, which was to last until 1938.