A foldable rigid frame attachment system for portable, inflatable pontoon boats including first and second inflatable elongated pontoons arranged to be approximately parallel to each other. The pontoons include pairs of upwardly extending strips formed in a longitudinal direction on both sides of the centerline on the top of each of the pontoons. Transversal slats connect the pontoons, and the bottom of each slat includes engaging means to engage the strips of the pontoons. The pontoon boat is held rigid by first and second underbars. The first underbar is positioned in a longitudinal direction on the top of the first pontoon in between the pairs of strips and below the slats, the second underbar is positioned in a longitudinal direction on the top of the second pontoon in between the pairs of strips and below the slats. As the pontoon boat is inflated, the underbars are locked into position between the pontoons and the slats.