Cement-based photocatalytic composition, which comprises: (a) at least one cement binder; (b) at least one photocatalyst; (c) at least one cellulose ether; (d) at least one fluidizing agent; (e) at least one first calcareous filler in the form of particles of which at least 95% by weight has a size not greater than 100 μm; (f) at least one second calcareous filler in the form of particles of which at least 95% by weight has a size not greater than 30 μm; (g) at least one silane supported on an inorganic support in the form of powder. Such composition can be employed as a water paint for obtaining wall coatings with very low thickness, in particular the outdoor applications, which ensure a high and stable photocatalytic effect over time even with relatively low quantities of photocatalyst, generally lower than 10% by weight, with optimal results in terms of uniformity of the coating and resistance of the same to weathering agents.