In the sciences, an experimentum crucis, or critical experiment, is an experiment capable of decisively determining whether or not a particular hypothesis or theory is correct. In particular, such an experiment must typically be able to produce a predictable result that no established hypothesis or theory is capable of producing.

The production of such an experiment is considered necessary for a particular hypothesis or theory to be considered an established part of the body of scientific knowledge. It is not unusual in the history of science for theories to be developed fully before producing a critical experiment. A given theory which is in accordance with known experiment but which has not yet produced a critical experiment is typically considered worthy of exploration in order to discover such an experimental test.

A famous example in the 20th century of an experimentum crucis was the expedition led by Arthur Eddington to Principe Island in Africa in 1919 to record the positions of stars around the Sun during a solar eclipse. The observation of star positions confirmed predictions of gravitational lensing made by Albert Einstein in the general theory of relativity published in 1915. Eddington's observations were considered to be the first solid evidence in favor of the Einstein's theory.

In some cases, a proposed theory can account for existing anomalous experimental results for which no other existing theory can furnish an explanation. An example would be the ability of the quantum hypothesis, proposed by Max Planck in 1900, to account for the observed black-body radiation spectrum, an experimental result which the existing classical Rayleigh-Jeans law could not predict. Such cases are not considered strong enough to fully establish a new theory, however, and the case of quantum mechanics, it took the confirmation of the theory through new predictions for the theory to gain full acceptance.