An interior storm window assembly has an outer frame adapted to be attached to an existing window frame or wall and having a channel running around an interior surface thereof, and a window unit comprising an inner frame surrounding a window pane. The window unit is adapted to be snap-fit inside the outer frame so that the window unit is securely held within the outer frame without play. There is a locking device for locking the window unit within the outer frame. The outer frame is attached to an existing window frame or to a wall surface surrounding an existing window, to provide an extra layer of protection against hurricane-force winds.