Polyaniline (PANI)/CdO nanocomposite was prepared for the first time in aqueous diethylene glycol solution medium, by chemical oxidative polymerization, and its photocatalytical activity was studied. Optical analysis of the new PANI/CdO nanocomposite revealed that electron densities and bond energies of the PANI homopolymer decreased after modifying with CdO nanoparticles, due to interactions between PANI chains and CdO nanoparticles. The prepared PANI/CdO nanocomposite exhibits excellent photocatalytic activity under both UV light and natural sunlight irradiation. The photocatalytic decolorization rate was increased up to 7 times after CdO addition, compared to the decolorization rate of PANI homopolymer under UV light irradiation. During the photocatalytic activity investigations, methylene blue and malachite green dyes were photocatalytically decolorized under natural sunlight irradiation with 99% efficiency by the use of 0.4 mg/mL PANI/CdO nanocomposite as photocatalyst. Furthermore, the PANI/CdO photocatalyst retains its efficiency with slight decreases upon being reused up to five times.