Family resemblance (German: Familienähnlichkeit) is a philosophical idea made popular by Ludwig Wittgenstein, with the best known exposition given in his posthumously published book Philosophical Investigations (1953). It argues that things which could be thought to be connected by one essential common feature may in fact be connected by a series of overlapping similarities, where no one feature is common to all of the things. Games, which Wittgenstein used as an example to explain the notion, have become the paradigmatic example of a group that is related by family resemblances. It has been suggested that Wittgenstein picked up the idea and the term from Friedrich Nietzsche, who had been using it, as did many nineteenth century philologists, when discussing language families.

The first occurrence of the term family resemblance is found in Arthur Schopenhauer (1788–1860; The World As Will and Representation §§17, 27, 28) who attributed the term to the school developed by Friedrich Wilhelm Joseph von Schelling (1775–1854). The next occurrence appeared in a note from 1930, commenting on Oswald Spengler's ideas. The notion itself features widely in Wittgenstein's later work, and in the Investigations it is introduced in response to questions about the general form of propositions and the essence of language – questions which were central to Wittgenstein throughout his philosophical career. This suggests that family resemblance was of prime importance for Wittgenstein's later philosophy; however, like many of his ideas, it is hard to find precise agreement within the secondary literature on either its place within Wittgenstein's later thought or on its wider philosophical significance.

Since the publication of the Investigations, the notion of family resemblance has been discussed extensively not only in the philosophical literature, but also, for example, in works dealing with classification where the approach is described as "polythetic", distinguishing it from the traditional approach known now as "monothetic". Prototype theory is a recent development in cognitive science where this idea has also been explored. As the idea gained popularity, earlier instances of its occurrence were rediscovered e.g. in 18th-century taxonomy, in the writings of Lev Vygotsky or Władysław Tatarkiewicz.