The present study aimed to assess the effects of microplastics (MPs) and metal bioaccumulation on the bivalve species Donax trunculus Linnaeus, 1758, Cerastoderma edule (Linnaeus, 1758), Mya arenaria Linnaeus, 1758 and Mytilus galloprovincialis Lamarck, 1819 from the Bulgarian Black Sea coast and their adaptive capacity to pollution. The MPs accumulation in the bivalves was observed under a stereomicroscope after 10% KOH tissue digestion. Metal elements (Cd, Cu, Cr, Ni, Pb, Zn and Fe) in the bivalve soft tissues were measured by EPA-METHOD 3052. The effects of pollutants on the bivalve species were assessed by oxidative stress (OS) biomarkers, which were measured spectrophotometrically. MPs were observed in all studied species but at a different ratio, pellets being the most numerous (94.7%). The accumulated MPs and metal elements induced OS but significantly correlated with different OS indices in the individual bivalve species. The principal component analysis suggested that the MPs accumulation probably leads to changes in bivalve cells similar to those caused by Pb and associated with protein oxidation and glutathione levels. In conclusion, the accumulated MPs and metal elements caused OS in all studied bivalves, which, in turn, activated their antioxidant system. This suggested the presence of adaptive potential of the bivalve species to the current ecological state of the marine environment in their habitats of the Bulgarian Black Sea sublittoral.