Porro prism is a refractive prism used in optical instruments to modify the orientation of images. A single Porro prism can also be regarded as a roof prism, but it is not used in binoculars, so a variation of the double Porro prism is the Porro Abbe prism.

Porro prism is an isosceles right-angle prism made of glass blocks, with the end plane facing the right angle. In use, the light enters from the largest rectangular surface of the prism, undergoes two total reflections on the inclined surface, and then penetrates to the original incident plane and emits. This is because the light only enters and exits in a normal state, and the prism does not have a dispersion effect. However, the image entering the Porro prism will be flipped 180° and will be emitted in the original direction of entry. The light image after the Porro prism has been reflected twice, so the handedness does not change.

Porro prisms are usually used in pairs in the form of a double Porro prism, where the second prism is rotated 90° relative to the first. The net effect of the prism system is that the incident light is changed to a parallel direction, the image is rotated 180°, and the handedness remains unchanged. Double Porro prism systems are suitable for changing the image direction in small optical telescopes, especially in many binoculars to provide image reconstruction and longer optical path folding, effectively shortening the distance between the objective lens and the eyepiece. Usually in a double Porro prism combination, the two prisms are glued together and the excess parts are cut off to reduce weight and size.