The moss flora of Lake County, California comprises of 300 species and one variety within 111 genera. This represents 44 percent of the California moss flora occupying an area less than one percent of the state land area. Six species, Bryum veronense De Notaris, Gemmabryum kunzei (Hornschuch) J.R. Spence, Schistidium flaccidum (De Notaris) Ochyra, Tortella alpicola Dixon, Trichodon cylindricus (Hedwig) Schimper, and Trichostomum brachydontium Bruch were documented as new for California. Four species, Acaulon mediterraneum Limpricht, Funaria convexa Spruce, Gymnostomum viridulum Bridel and Schistidium echinatum Ignatova & H.H. Blom, were documented as new records for the North American moss flora. During this study, thirteen species were described as new to science with at least one occurrence documented from Lake County: Didymodon californicus J.A. Jiménez, D.R. Toren & Shevock, D. eckeliae R.H. Zander, D. norrisii R.H. Zander, Gemmabryum vinosum J.R. Spence & Kellman, Grimmia serrana Muñoz, Shevock & D.R. Toren, G. torenii R.I. Hastings, Homalothecium californicum Hedenäs, Huttunen, D.H. Norris & Shevock, Imbribryum torenii J.R. Spence & Shevock, Orthotrichum persimile F. Lara, R. Medina & Garilleti, Ptychostomum pacificum J.R. Spence & Shevock, Schistidium splendens T.T. McIntosh, H.H. Blom, D.R. Toren & Shevock, S. squarrosum T.T. McIntosh, H.H. Blom, D.R. Toren & Shevock and Scleropodium occidentale B.E. Carter.