A diverse fossil flora of middle to late Albian age occurs in the upper half of the Winthrop Formation of the Methow basin in north-central Washington State, USA. The formation represents a transition from marine to terrestrial conditions within what is inferred, on paleomagnetic and paleofloral grounds, to be an allochthonous tectonic terrane, known as the Baja BC block, which formed adjacent to western Mexico, about 3,000 km south of its present position. The fossil plants were deposited in a braided-stream setting, with the richest plant sites occurring just above incipient paleosols and within probable crevasse splays. The flora consists of approximately 145 species in the Hepaticopsida, Equisetopsida, Polypodiopsida, Pteridospermopsida, Cycadopsida, Ginkgopsida, Bennettitopsida, Pinopsida, and Magnoliopsida. Its floral diversity exceeds that of any other contemporaneous paleoflora in the Rocky Mountain region. Sixty percent of the species are angiosperms, with the ferns, the cycads, and the conifers each comprising from approximately 10% to 15% of the total. The present study treats the spore plants of the Winthrop Formation and is the first in a series of three papers to cover the complete flora. This first portion of the flora consists of one species of the Hepatcopsida; two of the Equisetopsida, including a remarkable occurrence of the relict genus Neocalamites; and 19 species of ferns, of which two belong to Blechnaceae, two to Dicksoniaceae, two to Gleicheniaceae, three to Matoniaceae, and 10 of uncertain relationship. Five species in this latter category belong in the problematical Mesozoic genus Cladophlebis. In the uncertain category, we also propose two new morphogenera for sterile fern leaves that will, hopefully, reduce some of the confusion caused by assigning fern leaves with such morphology to living genera. We describe 5 new genera and 10 new species of Pteridophytina here. Overall, the spore-plants of the Winthrop flora are concordant with those of other late Mesozoic floras from North America and Europe.