As grazers, sea urchins are keystone species in tropical marine ecosystems, and their loss can have important ecological ramifications. Die-offs of urchins are frequently described but their causesare often unclear, in part because systematic examinations of animal tissues at gross and microscopic level are not done. In some areas, urchins are being employed to control invasive marine algae. Here we describe the pathology of a mortality event in Tripneustes gratilla in Hawaii where urchins were being translocated to control invasive algae. Although we did not determine the cause of the mortality event, our investigation suggests that animals died from inflammation of the test and epidermal ulceration followed by inability to maintain coelomic fluid pressure, colonization of coelomic fluid by opportunists (diatom, algae), and inappetence. Parasites, bacteria, fungi, and viruses were not evident as a primary cause of death. Pathology was suggestive of an environmental cause such as food limitation or toxin, possibilities that could be more aggressively pursued in future investigations. These findings highlight the need for caution and additional tools to better assess health when translocating marine invertebrates to ensure maximal biosecurity.