In quantum mechanics, Schrödinger's cat is a thought experiment that illustrates a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in a closed box, as a result of its fate being linked to a random subatomic event that may or may not occur. This thought experiment was devised by physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein to illustrate what Schrödinger saw as the problems of the Copenhagen interpretation of quantum mechanics.

In Schrödinger's original formulation, a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor (e.g. a Geiger counter) detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison, which kills the cat. The Copenhagen interpretation implies that, after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality resolves into one possibility or the other.

Though originally a critique on the Copenhagen interpretation, Schrödinger's seemingly paradoxical thought experiment became part of the foundation of quantum mechanics. The scenario is often featured in theoretical discussions of the interpretations of quantum mechanics, particularly in situations involving the measurement problem. The experiment is not intended to be actually performed on a cat, but rather as an easily understandable illustration of the behavior of atoms. As a result, Schrödinger's cat has had enduring appeal in popular culture. Experiments at the atomic scale have been carried out, showing that very small objects may be superimposed; superimposing an object as large as a cat would pose considerable technical difficulties.

Fundamentally, the Schrödinger's cat experiment asks how long superpositions last and when (or whether) they collapse. Interpretations for resolving this question include that the cat is dead or alive when the box is opened (Copenhagen); that a conscious mind must observe the box (Von Neumann–Wigner); that upon observation the universe branches into a universe where the cat is alive, and one where it is dead (many-worlds); that every object (such as the cat, and the box itself) is an observer, but superposition is relative depending on the observer (relational); that superposition never truly exists due to time-travelling waves (transactional); that merely observing the box either slows or accelerates the cat's death (quantum Zeno effect); among other theories which assert that the cat is dead or alive long before the box is opened. It is unclear which interpretation is correct; the underlying issue raised by Schrödinger's cat remains an unsolved problem in physics.