The influence of crude oil and chemical dispersant was evaluated over planktonic bacteria and biofilms grown on API 5L steel surfaces in microcosm systems. Three conditions were simulated, an untreated marine environment and a marine environment with the presence of crude oil and a containing crude oil and chemical dispersant. The results of coupon corrosion rates indicated that in the oil microcosm, there was a high corrosion rate when compared with the other two systems. Analysis of bacterial communities by 16S rRNA gene sequencing described a clear difference between the different treatments. In plankton communities, theBacilliandGammaproteobacteriaclasses were the most present in numbers of operational taxonomic unit (OTUs). TheVibrionales,Oceanospirillales, andAlteromonadalesorders were predominant in the treatment with crude oil, whereas in the microcosm containing oil and chemical dispersant, mainly members ofBacillalesorder were detected. In the communities analyzed from biofilms attached to the coupons, the most preponderant class wasAlphaproteobacteria, followed byGammaproteobacteria. In the control microcosm, there was a prevalence of the ordersRhodobacterales,Aeromonadales, andAlteromonadales, whereas in the dispersed oil and oil systems, the members of the orderRhodobacteraleswere present in a larger number of OTUs. These results demonstrate how the presence of a chemical dispersant and oil influence the corrosion rate and bacterial community structures present in the water column and biofilms grown on API 5L steel surfaces in a marine environment.