The invention relates to a spiral-shaped photoreactor (10) comprising at least one plate (40) wound in a spiral shape about a central axis (x) and forming at least one spiral-shaped flow channel (30), wherein said flow channel (30) is connected to at least one pipe for the intake of a reaction mixture and to at least one pipe (35) for the discharge of said reaction mixture; characterised in that said flow channel (30) is delimited on one side by one face of the plate, which comprises a layer (41) of at least one photocatalyst, and on the other side by the other face of said plate, which comprises a means (51) for emitting photons at a wavelength suitable for activating said photocatalyst in the presence of the reaction medium in the flow channel (30).