The present invention provides an alkali metal salt of fluorosulfonyl imide having favorable heat resistance and a reduced content of specific impurities and a water content, and provides a method for producing an alkali metal salt of fluorosulfonyl imide, which is capable of easily removing a solvent from a reaction solution. An alkali metal salt of fluorosulfonyl imide of the present invention is represented by the following general formula (I) and has a mass loss rate of 2% or less when the alkali metal salt of fuluorosulufonyl imide is kept at 100° C. for 8 hours under an air current. A method for producing an alkali metal salt of fluorosulfonyl imide of the present invention comprises a step of concentrating a solution of the alkali metal salt of fulorosulfonyl imide by bubbling a gas into a reaction solution containing the alkali metal salt of fulorosulfonyl imide, and/or concentrating a solution of the alkali metal salt of fulorosulfonyl imide by thin layer distillation.