A novel extendible flexible flood barrier manufactured from textile and membrane materials has been developed which comprises a textile/membrane wall and support posts configured to be movable between a stowed position and a deployed position and wherein, in the deployed position, forms a leak-free barrier against flood water or other fluids. The flexible wall is stored in a container on one side of the opening to be protected and extended to the receiver on the opposite of the opening to deploy the barrier. Structural members, such as posts or a building itself will react to the load from the flood water impinging on the wall. Intermediate posts or bollards can be added to lengthen the wall so larger spans can be closed-off with the flexible wall. The flexible wall can be configured for use in many applications including entire buildings, building entrances, transit system passages, such as ventilation shafts or elevator shafts, stairwells, and other openings that can be threatened by flooding.