<p>Tortoise beetles (Cassidinae s. str., Chrysomelidae, Coleoptera) compose a highly diverse group of insects, with most species found in Neotropical regions, such as the Brazilian Atlantic Forest, one of the most important hotspots in the world. Neotropical cassidines are known to typically display strong host specialization and distinct temporal patterns; however, host plants of most species are still unknown and very few studies have quantitatively investigated their temporal distribution. Here, we present a comprehensive list of tortoise beetle species for Serra do Japi, an Atlantic Forest mountain range, by comparing and combining data from 26&#xa0;months of field work with data from previous samplings done in the same area. By collecting richness and abundance of adults for 12 consecutive months, as well as monthly mean temperature and precipitation, we also tested the correlation between assemblage and climatic factors. Our final list is composed of 71 cassidines species, with 11 new distribution records for the state of São Paulo and new host plant records for 36 of these species. We also show that the adults of this tortoise beetle assemblage present a seasonal pattern, similar to that of subtropical cassidines, which is significantly influenced by temperature variation, but not by precipitation. Our results highlight the importance of Serra do Japi as a conservation unit and contribute to a better understanding of several ecological aspects of Cassidinae <i>s. str.</i></p>