In botany, a photoassimilate is one of a number of biological compounds formed by assimilation using light-dependent reactions. This term is most commonly used to refer to the energy-storing monosaccharides produced by photosynthesis in the leaves of plants.

Photoassimilate movement through plants from "source to sink" using xylem and phloem is of biological significance. This movement is mimicked by many infectious particles - namely viroids - to accomplish long ranged movement and consequently infection of an entire plant.