Synedropsis abuflosensis sp. nov. is the third species of the genus recorded from a sub-tropical brackish water habitat as opposed to a polar distribution. This species is differentiated from others in the genus by having a different number of slits in the apical slit field at opposite ends of the valve (i.e., 4 at the end possessing a rimoportula versus 5 at the end lacking a rimoportula), an apically oriented rimoportula and 3-4 non-porous cingular bands. Its habitat is different than that of all known Synedropsis Hasle, Medlin & Syvertsen species as it is either benthic or epiphytic in algal mats rather than planktonic or associated with sea ice. The new species was found in waters with a conductivity of 7.1 µS ^cm-1 (salinity 3.3 psu) and a temperature of 33 °C. Morphological characters of the new species are described based on light and scanning electron microscopy observations. A comparison of S. abuflosensis sp. nov. with morphologically similar species is provided.