We present new Mo isotope data from the Mexican continental margin that, in conjunction with previous data, allow us to propose a mechanistic description of the Mo isotope system in marine sediments. We hypothesize that there are unique environmentally dependent Mo isotope signatures recorded in marine sediments that reflect the mechanisms responsible for authigenic Mo accumulation. Open-ocean anoxic sites, defined as having dissolved oxygen and sulfide concentrations near zero in the overlying water, exhibit a δ98/95Mo isotope signature of +1.6‰. We believe this value reflects Mo sulfide formation via diagenetic processes within sediments. Quantitative formation of Mo sulfide within the sulfidic water column of euxinic environments results in sediment isotope values similar to the modern seawater value (+2.3‰), as typified by samples from the highly sulfidic Black Sea. In contrast to these reducing settings, manganese oxide–rich sediments have measured Mo isotope values that are more negative (relative to seawater) than any other sediment samples analyzed to date, similar to Fe-Mn crusts (∼−0.7‰). Most measured Mo isotope compositions of marine sediments from open-ocean settings appear to reflect a mixture of lithogenic Mo (0.0‰) and the Mo signature of a specific authigenic Mo accumulation mechanism. We therefore suggest that Mo isotopes may record unique signatures that reflect the dominant chemical mechanism for Mo sequestration into sediments.