A colloidal suspension of nanoparticles of the conjugated organic molecule tetracene is made by injecting a small amount of concentrated tetracene/tetrahydrofaran solution into water. The resulting nanoparticles are characterized by atomic force microscopy, their optical absorption and emission spectra, and their fluorescence decay times and quantum yields. Their optical properties in room-temperature aqueous solution are similar to what is observed for bulk polycrystalline films of tetracene, indicating a substantial amount of crystallinity. Due to their high surface area, these organic semiconductor nanoparticles demonstrate photocatalytic activity toward several different organic dye molecules in aqueous solution. The kinetics and oxygen dependence of the reactions are consistent with a heterogeneous type I reaction, similar to what is observed for other semiconductor photocatalysts, and the quantum yields at high surface coverage and low light intensity are comparable to those of inorganic photocatalysts.