The photoreactor comprises a tube bundle (10) made up of numerous capillary tubes (11) through which a reaction medium flows. The tubes (11) are transparent. Solar radiation or artificial radiation acts upon the reaction medium for effecting a photochemical or photobiological treatment. The inlet chamber (12) connected with the tube bundle (10) comprises a flow distributor (16) which distributes the reaction medium from the fluid inlet (15) to the tubes (11). The flow distributor (16) allows for a smaller volume of the inlet chamber (12). The reactor volume, which is not irradiated, is thus reduced, and the efficiency of the rector is enhanced.