A new species of Sinamia is described from fluvial-lacustrine deposits in southern Gansu Province, China. The type material consists of multiple specimens that were recovered in three large blocks, two containing over 60 individuals preserved on a single bedding plane. In addition, material from two other localities is referred to this taxon. One of these contains disarticulated remains of numerous individuals preserved in a single bedding plane. Material from the other locality, from a different region of Gansu, was collected years previously and consists of a block with multiple specimens previously attributed to Sinamia zdanskyi. We interpret the preservation of specimens from all three localities as resulting from the fish becoming isolated in a restricted area, possibly made seasonally inhabitable during flooding, that then was cut off from the main channel forming a small temporary lake or pond that eventually dried out, leaving the fish to be concentrated in a steadily shrinking water body. The covering sediments that preserved these fish were possibly deposited by a subsequent flood not long after the mass-death event. The type locality includes only Sinamia, but the other two localities include remains of a small turtle. The locality containing disarticulated remains also includes isolated crocodile and dinosaur teeth, suggesting that this locality is a crevasse splay. A phylogenetic analysis including the new species supports the monophyly of Sinamiidae, but not the monophyly of Sinamia.