During the past three decades, significant advances have been made in understanding how environmental factors modify the bioavailability and the toxicity of metals such as copper in aquatic environments. Several of these advances have led to the development of guidelines to indirectly account for modifying factors, adjustment of criteria on a site-specific basis, and direct changes to the U.S. Environmental Protection Agency (U.S. EPA) freshwater quality criteria. To date, most of this effort has focused on freshwater systems, although similar modifying factors exist in marine environments as well. This paper focuses on one such modifying factor, dissolved organic carbon (DOC), and describes a method to aid in risk assessments or to refine the saltwater copper criteria on a site-specific basis. The relationship between DOC and toxicity of copper to the most sensitive saltwater genus in the U.S. EPA criteria database, Mytilus, is extensively analyzed. Dissolved copper 50% effective concentrations (EC50s) are highly correlated (r² = 0.71, n = 54, p < 0.001) across a wide range of sample DOC concentrations (0.3–10 mg carbon [C]/L) and are explained by the equation EC50 = 11.53DOC^0.54. Two equations based on DOC are proposed for consideration as a means for deriving site-specific final chronic criteria (FCC) and final acute criteria (FAC) for copper in marine and estuarine environments (Copper FCC_DOC = 3.71DOC^0.54 and Copper FAC_DOC = 5.843DOC^0.54).