This study, accomplished is focused on the spatial and temporal distribution and species composition of phytoplankton in the southwestern Caspian Sea between January 2008 and December 2008. Samples were collected from 11 stations along three transects known as Lisar, Anzali and Sefidrood. This study identified 43 phytoplankton species comprised of diatoms (25 species), dinoflagellates (7 species), chlorophytes (6 species), cyanophytes (4 species), and euglenoids (1 species) were identified. The study showed, diatom Dactyliosolen fragilissimus (Bergon) Hasle in Hasle & Syvertsen 1996 and the dinofllagelate Prorocentrum cordatum (Ostenfeld) Dodge, 1975 numerically dominate the southwestern Caspian Sea. The annual average phytoplankton density was calculated as 2.35E+04±1.63E+04 Cell L^-1, with the maximum value recorded in the winter (4.50E+04 ± 3.90E+04 Cell L^-1). Diatoms were dominant during the autumn and winter (77.0% and 96.0%, respectively), while dinoflagellates were prevalent during the spring and summer (89.0% and 86.0%, respectively). The annual average chlorophyll a was calculated as 9.26±3.54 µg L^-1, with the maximum values were recorded in the winter (16.9±5.60 µg L-1). This is supported by the Spearman rank correlation result between phytoplankton abundance and chlorophyll a (r=0.551). The high phytoplankton cell density and chlorophyll a concentration coincided with the poor species composition.