Floristic sampling was carried out in Serra do Japi Biological Reserve, Jundiaí, state of São Paulo, Brazil, including physiognomies of semideciduous montane forest, a type of forest formation within the Atlantic Forest domain. Six hundred and sixty one species distributed in 129 families were recorded; the ten most diverse phanerogam families were: Asteraceae (56 species), Fabaceae (37), Rubiaceae (29), Poaceae (25), Solanaceae (24), Cyperaceae (19), Euphorbiaceae (19), Orchidaceae (19), Melastomataceae (17), and Piperaceae (17). The addition of 322 species from other floristic surveys increased the number of species of Serra do Japi to 976, distributed in 141 families. Compared to other floristic surveys carried out in the Atlantic Forest, our results highlight the floristic diversity of the study area and the importance of preserving the vegetation of this highly urbanized region.