Miniaturization is a key aspect for many technological applications and the use of microreactors is an excellent solution for the intensification of chemical processes for a variety of applications. However, standard microfabrication requires large facilities and intricate fabrication protocols, and consequently it is not easily available, generally resulting in high production costs. Herein, we present a very cheap, fast, and easy microreactor design for photocatalytic applications based on direct fused filament three-dimensional printing as a facilitating and widespread technology. The microreactor consists of three bodies directly printed in ABS (acrylonitrile butadiene styrene): a main body with a serpentine microchannel pattern where the photocatalyst is placed, a top holder with a transparent polymer window, and a base to clamp the parts. Several microreactor units were coated with TiO2 doped with Cu (2.4 wt %) nanoparticles synthesized by flame spray pyrolysis and tested for the photocatalytic degradation of two water pollutants showing excellent performance.