A de facto standard is a custom or convention that has achieved a dominant position by public acceptance or market forces (for example, by early entrance to the market). De facto is a Latin phrase (literally "in fact"), here meaning "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established".

The term de facto standard is used in contrast with standards defined by organizations or set out in law (also known as de jure standards), or to express the dominant voluntary standard when there is more than one standard available for the same use.

In social sciences a voluntary standard that is also a de facto standard is a typical solution to a coordination problem. The choice of a de facto standard tends to be stable in situations in which all parties can realize mutual gains, but only by making mutually consistent decisions. In contrast, an enforced de jure standard is a solution to the prisoner's problem.