In mathematics and physics, a phase space of a dynamical system is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usually consists of all possible values of position and momentum variables. The concept of phase space was developed in the late 19th century by Ludwig Boltzmann, Henri Poincaré, and Willard Gibbs.

Phase space can refer to:

• Phase space, a concept in physics, frequently applied in thermodynamics, statistical mechanics, dynamical systems, symplectic manifolds and chaos theory
• Phase Space (book), a collection of thematically-linked short stories in the Manifold Trilogy by Stephen Baxter
• Phase Space (album), an album by Steve Coleman and Dave Holland