In chemistry, Le Chatelier's principle (pronounced UK: /lə ʃæˈtɛljeɪ/ or US: /ˈʃɑːtəljeɪ/) is a principle used to predict the effect of a change in conditions on chemical equilibrium. Other names include Chatelier's principle, Braun–Le Chatelier principle, Le Chatelier–Braun principle or the equilibrium law.

The principle is named after French chemist Henry Louis Le Chatelier who enunciated the principle in 1884 by extending the reasoning from the Van 't Hoff relation of how temperature variations changes the equilibrium to the variations of pressure and what's now called chemical potential, and sometimes also credited to Karl Ferdinand Braun, who discovered it independently in 1887. It can be defined as:

If the equilibrium of a system is disturbed by a change in one or more of the determining factors (as temperature, pressure, or concentration) the system tends to adjust itself to a new equilibrium by counteracting as far as possible the effect of the change

— Le Chatelier's principle, Merriam-Webster Dictionary

In scenarios outside thermodynamic equilibrium, there can arise phenomena in contradiction to an over-general statement of Le Chatelier's principle.

Le Chatelier's principle is sometimes alluded to in discussions of topics other than thermodynamics.

Le Chatelier–Braun principle analyzes the qualitative behaviour of a thermodynamic system when a particular one of its externally controlled state variables, say ${\displaystyle L,}$ changes by an amount ${\displaystyle \Delta L,}$ the 'driving change', causing a change ${\displaystyle \delta _{\mathrm {i} }M,}$ the 'response of prime interest', in its conjugate state variable ${\displaystyle M,}$ all other externally controlled state variables remaining constant. The response illustrates 'moderation' in ways evident in two related thermodynamic equilibria. Obviously, one of ${\displaystyle L,}$ ${\displaystyle M}$ has to be intensive, the other extensive. Also as a necessary part of the scenario, there is some particular auxiliary 'moderating' state variable ${\displaystyle X}$, with its conjugate state variable ${\displaystyle Y.}$ For this to be of interest, the 'moderating' variable ${\displaystyle X}$ must undergo a change ${\displaystyle \Delta X\neq 0}$ or ${\displaystyle \delta X\neq 0}$ in some part of the experimental protocol; this can be either by imposition of a change ${\displaystyle \Delta Y}$, or with the holding of ${\displaystyle Y}$ constant, written ${\displaystyle \delta Y=0.}$ For the principle to hold with full generality, ${\displaystyle X}$ must be extensive or intensive accordingly as ${\displaystyle M}$ is so. Obviously, to give this scenario physical meaning, the 'driving' variable and the 'moderating' variable must be subject to separate independent experimental controls and measurements.

The principle can be stated in two ways, formally different, but substantially equivalent, and, in a sense, mutually 'reciprocal'. The two ways illustrate the Maxwell relations, and the stability of thermodynamic equilibrium according to the second law of thermodynamics, evident as the spread of energy amongst the state variables of the system in response to an imposed change.