The Red Sea is a unique marine ecosystem with contrasting gradients of temperature and salinity along its north to south axis. It is an extremely oligotrophic environment that is characterized by perpetual year-round water column stratification, high annual solar irradiation, and negligible riverine and precipitation inputs. In this study, we investigated whether the contemporary environmental conditions shape community assemblages by pyrosequencing 16S rRNA genes of bacteria in surface water samples collected from the northeastern half of this water body. A combined total of 1,855 operational taxonomic units (OTUs) were recovered as free-living and particle-attached bacterioplankton. Here, a few major OTUs affiliated with Cyanobacteria and Proteobacteria accounted for ~93% of all sequences whereas a tail of “rare” OTUs represented most of the diversity. OTUs allied to Surface 1a/b SAR11 clades and Prochlorococcus related to the high light-adapted (HL2) ecotype were the most widespread and predominant sequence types. Interestingly, the frequency of taxa that are typically found in the upper mesopelagic zone was significantly elevated in the northern transects compared to those in the central, presumably as a direct effect of deep convective mixing in the Gulf of Aqaba and waters exchange with the northern Red. Although temperature was the best predictor of species richness across all major lineages, both spatial and environmental distances correlated strongly with genetic distances. Our results suggest that the bacterial diversity of the Red Sea is as high as in other tropical seas and provide evidence for fundamental differences in the biogeography of pelagic communities between the northern and central regions.