A new diatom species, Staurosirella acidophila, is described from surface sediments and phytoplankton samples from tropical reservoirs in São Paulo, Brazil. Description of the new taxon is based on morphological characteristics observed using light (LM) and scanning electron microscopy (SEM). The new species is characterized by having rhombic-lanceolate valves with acuminate to rostrate ends, striae uniseriate, uninterrupted from valve face to valve mantle and areolae elongated, slit-like. Double line of incipient spines is for the first time reported for the genus Staurosirella. It belongs to the group of species morphologically related to Staurosirella pinnata sensu auct. non null. and shares some resemblance with species currently belonging to the genus Punctastriata. Comparison with similar species currently ascribed to the genera Punctastriata, Staurosira and Staurosirella are provided. The new species is described from sites that represent good ecological status (low nutrient concentrations and low pH) for the Metropolitan Region of São Paulo. Relative abundances and autecological information complement the present study, which is a contribution to the knowledge of diatoms thriving in environments with good ecological conditions in Brazil.