An object of the present invention is to provide a wet spray-on fireproofing composition with superior fire resistance properties, which is highly fluid during pumping and has superior pumping efficiency, yet ceases to be fluid after being sprayed on, enabling thick application, and which has no risk of cracking or lifting away from the surface of steel work due to shrinking despite containing no mineral fibers. The present invention is characterized in that in a wet spray-on fireproofing composition for spray-on application after merging and mixing a slurry component and an aluminum sulfate component in aqueous solution in a spraying nozzle, the total solid content of the slurry component includes as essential compositional components: (a) 20 percent by weight or more of gypsum; (b) 5 to 40 percent by weight of slaked lime and/or quick lime; (c) organic fibers and/or light-weight fillers; and water; and the content of elements which enhance fire resistance properties after drying is 20 percent by weight or more.