An adult, hermaphroditic Tridacna crocea ornamental clam imported from Vietnam into the USA became terminally moribund with sloughed byssal tissue and incomplete extension of the poorly responsive mantle and was necropsied. Necropsy findings included emaciation, visceral mass edema, and rare multifocal, 1-mm diameter, off-white to light tan gill nodules. Histopathology revealed marked inflammation and necrosis within the visceral mass and gills, with interstitial edema and atrophy of glandular, gonadal, and muscular tissues. Inflamed tissues contained large numbers of 10–15 μm extracellular, spherical organisms with a signet-ring morphology consistent with Perkinsus spp. trophozoites. The organisms often formed clusters of two to four cells and were surrounded by a host reaction consisting of a 1–4 μm rim of amorphous eosinophilic material and two to four host hemocytes. Incubation of infected host tissues in alternative Ray's fluid thioglycollate medium (ARFTM) confirmed the presence of Perkinsus sp. hypnospores that stained blue-black with Lugol's iodine. Polymerase chain reaction assays with sequencing of products revealed a high level of nucleotide similarity, but no exact match, to known P. olseni isolates. Perkinsus sp. organisms, including P. olseni and P. marinus, which are internationally reportable, are highly pathogenic destructive protozoa capable of disrupting ecosystems populated by naïve mollusks within the USA and negatively affecting both domestic and international shellfish industries. This is the first report of an exotic Perkinsus sp. pathogen in an imported ornamental clam maintained long term in a home aquarium. However, ongoing research indicates that T. crocea from Vietnam are commonly infected by such organisms. Veterinarians, aquarium facility mangers, and veterinary clients with hobby aquariums should use appropriate caution and responsible disposal practices for clam carcasses and for water in which imported ornamental clams have been housed. Such practices will reduce the possibility of dispersing viable, exotic Perkinsus sp. organisms into domestic waters.