Disclosed is a prefabricated modular kit, reversibly assemblable, adapted for the quick installation of a bathroom, including: A) lower module with: toilet bowl; discharge device; roll-holder; container; tiltable door; sprayhead; water connections; discharge duct; B) intermediate module with: sink surface; faucet; mixer; soap dispenser; hands-dryer; C) upper module with: container; foldable arm with showerhead; D) floor module with shower plate; E) ceiling module with light and exhaust fan; F) side wall module; G) door; the modules having the systems ready for connecting between the modules themselves and between the modules and the pre-existing wall.