Dementia praecox (sometimes loosely translated as premature dementia) is a theorized disorder of the brain, which in the late 19th century was identified as characteristically occurring in the young. An understanding of dementia praecox is vital for those seeking to fully understand the historical development of the concept of schizophrenia, though they cannot be considered synonyms.

The concept of dementia praecox is most closely associated with the work of Emil Kraepelin. In the fourth edition of his 1893 textbook Psychiatry: A Textbook for Students and Doctors, under “Psychic Degeneration Processes,” Kraepelin indexes three conditions that he would ultimately merge into one disorder: