A structure for blocking outflow of a fluid for a washing machine includes a door installed at an outlet of a detergent supply unit such that the door opens the outlet when wash water and detergent are supplied from the detergent supply unit to a washing tub and closes the outlet of the detergent supply unit when foam or steam is discharged from the washing tub to the detergent supply unit, thereby preventing the fluid, such as foam or steam in the washing tub, from flowing to the detergent supply unit through a water supply bellows.