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In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted fashion. Pericyclic reactions are usually rearrangement reactions. The major classes of pericyclic reactions are:

• Electrocyclic reactions
• Cycloadditions
• Sigmatropic reactions
• Group transfer reactions
• Cheletropic reactions

In general, these are considered to be equilibrium processes, although it is possible to push the reaction in one direction by designing a reaction by which the product is at a significantly lower energy level; this is due to a unimolecular interpretation of Le Chatelier's principle. Pericyclic reactions often have related stepwise radical processes associated with them. Some pericyclic reactions, such as the [2+2] cycloaddition, are 'controversial' because their mechanism is not definitively known to be concerted (or may depend on the reactive system). Pericyclic reactions also often have metal-catalyzed analogs, although usually these are also not technically pericyclic, since they proceed via metal-stabilized intermediates, and therefore are not concerted.

The 'largest' pericyclic reaction was the [14 pi] ring closure on a corrin performed by Albert Eschenmoser.

Due to the principle of microscopic reversibility, there is a parallel set of "retro" pericyclic reactions, which perform the reverse reaction.

Pericyclic reactions also occur in several biological processes.

• Claisen rearrangement of chorismate to prephenate in almost all prototrophic organisms.
• [1,5]-sigmatropic shift in the transformation of precorrin-8x to hydrogenobyrinic acid
• non-enzymatic, photochemical electrocyclic ring opening and a (1,7) sigmatropic hydride shift in vitamin d synthesis.
• a conversion of Isochorismate into salicylate and Pyruvate in a catalyzed, true pericyclic reaction ^{[1] [2]}.

• Woodward-Hoffmann rules