Abundance and diversity of microorganisms, including H2S-producing and corrosive bacteria, in the produced water collected from different locations at a Central Arabian Gas-Oil Separation Plant (GOSP) were examined by culture-based and culture-independent methods. Conventional MPN techniques showed that abundance of general heterotrophic bacteria exceeded acceptable levels more frequently in water samples from High Pressure-Production Trap (HPPT) and Water-Oil Separator (WOSEP) than in water from Production Header (PH). MPN assay of corrosive sulfidogenic sulfate-reducing bacteria (SRB) and thiosulfate-reducing bacteria (TRB) exceeded acceptable levels in 11% of samples. Cellular ATP and qPCR tests confirmed that bioburden and SRB exceeded acceptable levels more frequently in samples from HPPT and WOSEP than in the PH water, for instance, SRB exceeded acceptable levels in 67% of PH samples and 89% of samples from HPPT and WOSEP. Profiling microbial population using 16S rRNA gene amplicon sequencing revealed that microorganisms in the PH water differed significantly from microbial community in the HPPT and WOSEP samples. Bacterial amplicons predominated over archaeal in all examined samples. Microorganisms in the PH water were chiefly represented by aerobic heterotrophic bacteria and fermenters. In contrast, the HPPT and WOSEP samples contained a diverse bacterial population specialized primarily on the sulfur redox reactions. In conclusion, this study demonstrated that processing of produced fluids at the GOSP prompted the increase in H2S-producing and corrosive bacteria in the water samples from HPPT and WOSEP. An effective microbial monitoring and control programs for oil-producing facilities have to integrate multiple lines of evidence on the bacterial abundance, activity and diversity in order to prevent progressing of microbial souring and corrosion through the water handling facilities and reservoir.