Jean William Fritz Piaget (UK: /piˈæʒeɪ/, US: /ˌpiːəˈʒeɪ, pjɑːˈʒeɪ/; French: [ʒɑ̃ pjaʒɛ]; 9 August 1896 – 16 September 1980) was a Swiss psychologist known for his work on child development. Piaget's theory of cognitive development and epistemological view are together called genetic epistemology.

Piaget placed great importance on the education of children. As the Director of the International Bureau of Education, he declared in 1934 that "only education is capable of saving our societies from possible collapse, whether violent, or gradual". His theory of child development has been studied in pre-service education programs. Nowadays, educators and theorists working in the area of early childhood education persist in incorporating constructivist-based strategies.

Piaget created the International Center for Genetic Epistemology in Geneva in 1955 while on the faculty of the University of Geneva, and directed the center until his death in 1980. The number of collaborations that its founding made possible, and their impact, ultimately led to the Center being referred to in the scholarly literature as "Piaget's factory".

According to Ernst von Glasersfeld, Piaget was "the great pioneer of the constructivist theory of knowing". His ideas were widely popularized in the 1960s. This then led to the emergence of the study of development as a major sub-discipline in psychology. By the end of the 20th century, he was second only to B. F. Skinner as the most-cited psychologist.

Piaget was born in 1896 in Neuchâtel, in the Francophone region of Switzerland. He was the oldest son of Arthur Piaget (Swiss), a professor of medieval literature at the University of Neuchâtel, and Rebecca Jackson (French). Rebecca Jackson came from a prominent family of French steel foundry owners. Piaget was a precocious child who developed an interest in biology and the natural world. His early interest in zoology earned him a reputation among those in the field after he had published several articles on mollusks by the age of 15.

When he was 15, his former nanny wrote to his parents to apologize for having once lied to them about fighting off a would-be kidnapper from baby Jean's pram. There never was a kidnapper. Piaget became fascinated that he had somehow formed a memory of this kidnapping incident, a memory that endured even after he understood it to be false.

He developed an interest in epistemology due to his godfather's urgings to study the fields of philosophy and logic. He was educated at the University of Neuchâtel, and studied briefly at the University of Zürich. During this time, he published two philosophical papers that showed the direction of his thinking at the time, but which he later dismissed as adolescent thought. His interest in psychoanalysis, at the time a burgeoning strain of psychology, can also be dated to this period.

Piaget moved from Switzerland to Paris after his graduation and he taught at the Grange-Aux-Belles Street School for Boys. The school was run by Alfred Binet, the developer of the Binet-Simon test (later revised by Lewis Terman to become the Stanford–Binet Intelligence Scales). Piaget assisted in the marking of Binet's intelligence tests. It was while he was helping to mark some of these tests that Piaget noticed that young children consistently gave wrong answers to certain questions. Piaget did not focus so much on the fact of the children's answers being wrong, but that young children consistently made types of mistakes that older children and adults managed to avoid. This led him to the theory that young children's cognitive processes are inherently different from those of adults. Ultimately, he was to propose a global theory of cognitive developmental stages in which individuals exhibit certain common patterns of cognition in each period of development.