Serra do Mar State Park (PESM) is located in southeastern São Paulo state, Brazil, and is the largest Atlantic Rainforest conservation area in the country. The park is divided into 10 nuclei, and the Itutinga-Pilões nucleus (NIP) was selected for study since no survey of liverworts species richness had yet been conducted there. The floristic similarities between the NIP and different areas of Atlantic Rainforest in São Paulo state were also assessed. One hundred and eighty liverworts species were identified in the three vegetation types (lowland, submontane, and montane ombrophilous forests) encountered in the NIP, distributed among 62 genera and 21 families. Lejeuneaceae showed the highest species richness with 80 species. The high number of epiphyllous species found in the study area (26% of all species) was notable. In terms of their worldwide distributions, 57% of the species are Neotropical. Clustering analysis showed that the areas of the PESM and Jureia-Itatins Ecological Station were grouped together with the highest similarity values. The liverworts flora of the NIP demonstrates the importance of that nucleus for the conservation of liverworts diversity in the Atlantic Rainforest of the São Paulo state, as well as in Brazil.