In organic chemistry, a pericyclic reaction is a type of reaction where the transition state of molecule has a cyclic geometry. A pericyclic reaction mechanism can be found in:

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

Generally these are considered to be equilibrium processes, although it is possible to push the reaction in one direction by designing a reaction where the product is at a significantly lower energy level; this is due to a unimolecular interpretation of Le Chatelier's principle.

Formally, pericyclic reactions are concerted reactions, though often they have related stepwise radical processes associated with them. Some pericyclic reactions are 'controversial' (such as the 2 + 2 cycloaddition) because their mechanism is not known to be 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.