The moss flora of the Cascade-Siskiyou National Monument, located on the border of southwest Oregon and northwest California, comprises 201 moss species and one variety within 89 genera and 37 families. The majority of the Cascade-Siskiyou National Monument is in Oregon and it comprises less than 0.2 percent of the state; however, roughly 38 percent of moss species known to Oregon were found. Forty species recorded during work for this manuscript are rare or under-collected in Oregon and six are newly reported to the state. Species reported new to Oregon include Grimmia attenuata (Müll.Hal. & Kindb.) Kindb., Meesia longiseta Hedw., Physcomitrella readeri (Müll.Hal.) I.G.Stone & G.A.M.Scott, Pseudocalliergon angustifolium Hedenäs, Schistidium splendens T.T.McIntosh, H.H.Blom, D.R.Toren & Shevock, and Weissia ligulifolia (E.B.Bartram) Grout.