There are 323 species and varieties of mosses in Missouri. Their distributions within the state are given at the county level based on Missouri collections examined by the author (deposited in ALTA, MO, NY, PH, S, SMS, UBC, UMO, US) as well as a literature survey on mosses reported from Missouri. The great majority of the collections examined in this study are deposited at the Missouri Botanical Garden (MO). The genera in the checklist are arranged alphabetically; within each genus the species and varieties are arranged alphabetically. Generic family placement is indicated (in parentheses) after each genus entry. A separate listing of the Missouri moss families and their genera, arranged alphabetically, is included. Twenty-one mosses were newly documented for Missouri during this study.