A survey of the bryophyte flora, Anthocerotophyta (hornworts), Marchantiophyta (liverworts), and Bryophyta (mosses) of Jefferson County, Ohio was conducted from 1998 to 2004. Jefferson County (106,416 ha) is located in the unglaciated Allegheny Plateau region. Its eastern border is the Ohio River. Approximately 20% of the county has been mined for coal since the early 1900’s, and 29% has been cleared for agriculture. Seventeen (17) species of liverworts and 1 species of hornwort were found. Jefferson County records for moss species increased from 0 to 115. The moss flora of Jefferson County is compared to that of other Ohio counties using S0renson’s similarity index (SI).