An object of the present invention is to provide a fiber treatment agent that is capable of forming on fibers a film that has excellent water resistance and excellent adhesion to a matrix resin. The present invention provides a fiber treatment agent comprising an aqueous medium, a water-insoluble polyamide dispersed in the aqueous medium, and a water-soluble polyamide that is present in an amount of 2 to 50 parts by mass per 100 parts by mass of the water-insoluble polyamide.