Stochastic music (Also known as: Stochastism, Stochasticism) is a method of composition in 20th century classical music developed by Iannis Xenakis and described in his book Formalized Music. This music arose from Xenakis' critique of Serialism, which in Xenakis' point of view just substituted the natural causality of tonal music by the stricter abstract causality in order to create atonal music. However, Xenakis concludes, this causality can't be perceived and what the listener hears is just a random set of tones, which causes that Serialism crushes under its complexity.

Xenakis meant to create music that would evade bindings of causality but would remain logical, and its inherent logic would be perceivable on the macroscopic level. This often ends up in a slowly evolving mass of sound. "Stochastic" is term borrowed from theory of probability and Xenakis describes it as "an asymptotic evolution towards a stable state, towards a kind of goal, of stochos". Stochastic music is kind of guided indeterminism, where the following state is only partially determined by the preceding state, that means the concrete state n+2 follows after the state n+1 only with some probability. Every aspect of music, be it pitch, timbre, dynamics and so on can be subordinated to such laws of chance.

Stochastic music heavily borrows from mathematics such as law of large numbers, probability theory, game theory, Boolean algebra, Markovian chain, Poisson law, group theory and so on. This mathematical means are used to guide the indeterminism. This method is different to Indeterminacy, where the randomness is not guided by mathematical or other such laws. Also, the output of stochastic processes is usually fixed in the traditional score in the end, but Indeterminacy often uses open scores and even alternative notations.

Stochastic music is sometimes referred to as a compositional technique, sometimes as a genre. Though Stochastic music borrows heavily from both Serialism and Indeterminacy, it distinguishes itself from either through its core philosophical ideas, mainly the idea that application of scientific, or other non-musical notions of causality onto music can enrich our perception of music and redefine the concept of "harmony".

Because of the need of a lot of calculations that take a lot of time when done by hand, usage of computers is also welcomed in stochastic music. That led to scores created by computers using preset algorithms, as well as to purely electronic pieces.

Stochastic music has a very few followers, most notable being James Tenney.