Alois Alzheimer (/ˈɑːltshaɪmər/ AHLTS-hy-mər, German: [ˈaːlɔɪs ˈʔaltshaɪmɐ]; 14 June 1864 – 19 December 1915) was a German psychiatrist, neuropathologist and colleague of Emil Kraepelin. He is credited with identifying the first published case of "presenile dementia", which Kraepelin later identified as Alzheimer's disease.

Alzheimer was born in Marktbreit, Bavaria, on 14 June 1864, the son of Anna Johanna Barbara Sabina and Eduard Román Alzheimer. His father served in the office of notary public in the family's hometown. The family was devoutly Catholic.

The Alzheimers moved to Aschaffenburg when Alois was still young in order to give their children an opportunity to attend the Royal Humanistic Gymnasium (high school). After graduating with Abitur in 1883, Alzheimer studied medicine at University of Berlin, University of Tübingen, and University of Würzburg. In his final year at university, he was a member of a fencing fraternity, and even received a fine for disturbing the peace while out with his team. In 1887, Alzheimer graduated from Würzburg as Doctor of Medicine.

In 1888, Alzheimer spent five months assisting mentally ill women before he took an office in the city mental asylum in Frankfurt, the Städtische Anstalt für Irre und Epileptische (Asylum for Lunatics and Epileptics). Emil Sioli [de], a noted psychiatrist, was the dean of the asylum. Another neurologist, Franz Nissl, began to work in the same asylum with Alzheimer. Together, they conducted research on the pathology of the nervous system, specifically the normal and pathological anatomy of the cerebral cortex. Alzheimer was the co-founder and co-publisher of the journal Zeitschrift für die gesamte Neurologie und Psychiatrie, though he never wrote a book that he could call his own.

While at the Frankfurt asylum, Alzheimer also met Emil Kraepelin, one of the best-known German psychiatrists of the time. Kraepelin became a mentor to Alzheimer, and the two worked very closely for the next several years. When Kraepelin moved to Munich to work at the Royal Psychiatric Hospital in 1903, he invited Alzheimer to join him.

At the time, Kraepelin was doing clinical research on psychosis in senile patients; Alzheimer, on the other hand, was more interested in the lab work of senile illnesses. They faced many challenges involving the politics of the psychiatric community. For example, formal and informal arrangements were made among psychiatrists at asylums and universities to receive cadavers.

In 1904, Alzheimer completed his habilitation at the Ludwig-Maximilians-Universität München (LMU), where he was appointed as a professor in 1908. Afterward, he left Munich for the Silesian Friedrich Wilhelm University in Breslau in 1912, where he accepted a post as professor of psychiatry and director of the Neurologic and Psychiatric Institute. His health deteriorated shortly after his arrival so that he was hospitalized. He died three years later.

Alzheimer is known for having a variety of medical interests including vascular diseases of the brain, early dementia, brain tumors, forensic psychiatry and epilepsy.