Truxillic acids are any of several crystalline stereoisomeric cyclic dicarboxylic acids with the formula (C₆H₅C₂H₂(CO₂H)₂. They are colorless solids. These compounds are obtained by the [2 + 2] photocycloadditions of cinnamic acid where the two trans alkenes react head-to-tail. The isolated stereoisomers are called truxillic acids. The preparation of truxillic acids provided an early example of organic photochemistry.

These compounds are found in a variety of plants, for example in coca. Incarvillateine, an alkaloid from the plant Incarvillea sinensis, is a derivative of α-truxillic acid.

Upon heating, truxillic acids undergo cracking to give cinnamic acid.

Truxillic acid can exist in five stereoisomers.

Below are the five stereoisomers of truxillic acid, called alpha, gamma, epsilon, peri, and epi. These are shown both in a 2D skeletal diagram with stereocenters indicated and a 3D rendering of the structural geometry of the isomers themselves.