Sagenopteris williamsii is an extinct pteridosperm (seed fern) that is known from the late Jurassic and Early Cretaceous strata of the western interior of North America. It was first described as Chiropteris williamsii by J.S. Newberry in 1891, based on specimens from the Great Falls coal field in Montana.Newberry, J.S. 1891. The flora of the Great Falls Coal Field, Montana. American Journal of Science, 3rd series, vol. 41, p. 191-201 and plate 14. In 1956, it was referred to Sagenopteris by W.A. Bell based on additional specimens from western Canada.Bell, W.A. 1956. Lower Cretaceous floras of western Canada; Geological Survey of Canada, Memoir 285, p. 80-81 and plates 31, 33, 34, and 36.
