In mathematics, an uncountable set is a set which is not countable. Here, "countable" means countably infinite or finite, so by definition, all uncountable sets are infinite.

The best known example of an uncountable set is the set R of all real numbers; Cantor's diagonal argument shows that this set is uncountable. The diagonalization proof technique can also be used to show that several other sets are uncountable as well, for instance the set of all infinite sequences of natural numbers (and even the set of all infinite sequences consisting only of zeros and ones) and the set of all subsets of natural numbers.

Not all uncountable sets have the same size; the sizes of infinite sets are analyzed with the theory of cardinal numbers. The statement that R is the smallest uncountable set (in the sense that its cardinal number is the smallest uncountable cardinal number) is the continuum hypothesis; this hypothesis is independent from the ordinary axioms of set theory.

The Cantor set is an uncountable subset of R. The Cantor set is a fractal and has Hausdorff dimension greater than zero but less than one. (R has dimension one.) This is an example of the following fact: any subset of R of Hausdorff dimension strictly greater than zero must be uncountable.