An attemperator in the form of a steam desuperheater is disclosed herein and comprises a water tube adapted to be connected to a source of water under high pressure and a spray tube communicating with the water tube through a flow-control device for controlling the flow of water from the water tube to the spray tube. The spray tube is adapted to be located in a steam line for injecting cooling water into the steam when its temperature exceeds a predetermined temperature and accordingly is formed with one or more nozzles for injecting a spray of water travelling along an expanding swirling path into the steam, the water being atomized into small droplets which easily evaporate in the steam. Preferably, a plurality of nozzles are provided arranged in rows extending axially of the spray tube and the rows are circumferentially spaced apart. Also, the nozzles preferably communicate with the inside of the spray tube through a series of small ports. A piston movable axially within the spray tube controls the quantity of water discharged into the steam.