Ilya Ilyich Mechnikov (15 May [O.S. 3 May] 1845 – 15 July 1916), also spelled Élie Metchnikoff, was a zoologist from the Russian Empire of Moldavian noble ancestry best known for his research in immunology (study of immune systems) and thanatology (study of death). He and Paul Ehrlich were jointly awarded the 1908 Nobel Prize in Physiology or Medicine "in recognition of their work on immunity".

Mechnikov was born in a region of the Russian Empire that is today part of modern-day Ukraine to a Moldavian noble father and a Ukrainian-Jewish mother. He later moved to France. Given this complex heritage, five different nations and peoples lay claim to Metchnikoff. Despite having a mother of Jewish origin, he was baptized Russian Orthodox, although he later became an atheist.

Honoured as the "father of innate immunity", Metchnikoff was the first to discover a process of immunity called phagocytosis and the cell responsible for it, called phagocyte, specifically macrophage, in 1882. This discovery turned out to be the major defence mechanism in innate immunity, as well as the foundation of the concept of cell-mediated immunity, while Ehrlich established the concept of humoral immunity to complete the principles of immune system. Their works are regarded as the foundation of the science of immunology.

Metchnikoff developed one of the earliest concepts in ageing, and advocated the use of lactic acid bacteria (Lactobacillus) for healthy and long life. This became the concept of probiotics in medicine. Mechnikov is also credited with coining the term gerontology in 1903 for the emerging study of aging and longevity. In this regard, Ilya Mechnikov is called the "father of gerontology" (although, as often happens in science, the same title is sometimes applied to multiple people who contributed to aging research).

Supporters of life extension celebrate 15 May as Metchnikoff Day and use it as a memorable date for organizing activities.

Metchnikoff was born in the village of Ivanovka [uk], Kharkov Governorate, in the Russian Empire, now located in Kupiansk Raion, Kharkiv Oblast in Ukraine. He was the youngest of five children of Ilya Ivanovich Mechnikov, an officer of the Imperial Guard. His mother, Emilia Lvovna (Nevakhovich), the daughter of the writer Leo Nevakhovich, largely influenced him on his education, especially in science. The Nevakhovich family was Jewish.

The family name Mechnikov is a translation from Romanian, since his father was a descendant of the Chancellor Yuri Stefanovi, the grandson of Nicolae Milescu Spătarul. Yuri Stefanovich immigrated to Russia together with Dimitrie Cantemir in 1711 after the unsuccessful campaign of Peter I on the Danubian Principalities. For two and a half centuries, the Mechnikov family lived in St. Petersburg, where it became connected by family ties with many Russian princely families. The word "mech" is a Russian translation of the Romanian "spadă" (sword), which originated with Spătar (Sword-bearer). His elder brother Lev became a prominent geographer and sociologist.

In 1856, Metchnikoff entered the Kharkov Lycée, where he developed his interest in biology. Convinced by his mother to study natural sciences instead of medicine, in 1862 he tried to study biology at the University of Würzburg, but the German academic session would not start by the end of the year. Metchnikoff thus enrolled at Kharkov Imperial University for natural sciences, completing his four-year degree in two years.