We investigate the diversity of thalloid liverworts in Parque Nacional da Chapada das Mesas (PNCM). Illustrations, descriptions, taxonomic characters, and notes on the geographic distributions of species, as well as an identification key, are provided. We collected specimens during the rainy seasons between 2017 and 2020. Six families, six genera, and seven species were recorded, of which five species are recorded for the first time from Maranhão state, two species are recorded for the first time from the Brazilian Northeast Region, and two are newly recorded from the Brazilian Cerrado. Our results increase in the number of thallose liverwort species in Maranhão by 57%. Our data add to what is known about PNCM, Maranhão, and Brazilian Cerrado liverworts, and reinforce the importance of PNCM for conservation.