Determination of the spatial distribution of marine plankton off-coast of Sitra, Kingdom of Bahrain was conducted from August 2014-July 2015 to identify, categorize, and classify taxonomically the phyto-and-zooplankton.Population size was calculated based on abundance, density, and biomass from the collected water samples. Results showed 20 species of phytoplankton and 14 species of zooplankton. The phytoplankton includes 13 species which belong to Phylum Chrysophyta or diatoms, and seven species in Phylum Dinoflagellata or dinoflagellates. The 14 species of zooplankton were categorized to eight phyla-Protozoa, Cnidaria, Platyhelminthes, Rotifer, Annelida, Mollusca, Arthropoda, and Chordata. The identified species of phytoplankton and zooplankton vary in abundance, density, and biomass. Fragillariopsis and Navicula species were identified as the most abundant phytoplankton with higher density and biomass while Ceratium furca and Ceratium fusus of the zooplankton. Statistical analysis using ANOVA (Analysis of Variance) revealed significant differences at p = 0.05 on abundance and density, however, insignificant difference in biomass. Based on dispersion index, the phytoplankton and zooplankton were found to be vertically distributed and aggregated mostly on the upper column where sunlight is abundant. Surface water temperature and salinity significantly affect the spatial distribution and population size of the identified marine plankton.