Coastal ecosystems rely upon oyster reefs to filter water, provide protection from storms, and build habitat for other species. From a chemistry perspective, few details are available to illustrate how these shellfish construct such extensive reef systems. Experiments presented here show that oysters generate a biomineralized adhesive material for aggregating into large communities. This cement is an organic-inorganic hybrid and differs from the surrounding shells by displaying an alternate CaCO3 crystal form, a cross-linked organic matrix, and an elevated protein content. Emerging themes and unique aspects are both revealed when comparing oyster cement to the adhesives of other marine organisms. The presence of cross-linked proteins provides an analogy to mussel and barnacle adhesives whereas the high inorganic content is exclusive to oysters. With a description of oyster cement in hand we gain strategies for developing synthetic composite materials as well as a better understanding of the components needed for healthy coastal environments.