An adult female rainbow lorikeet (Trichoglossus haematodus haematodus) was presented with an acute onset of vomiting and melena. Fecal cytology and fecal Gram stain confirmed the presence of erythrocytes and bacteria morphologically similar to clostridia. Clostridium tertium was cultured from the feces. Leukocytosis with moderate absolute heteropenia, absolute lymphocytosis, and toxic immature heterophils was observed on the complete blood count. Plasma biochemistry analysis included mild hyperuricemia and metabolic alkalosis. The most significant radiographic abnormalities included proventricular filling defects, intestinal dilatation, and gas accumulation in the gastrointestinal tract. All radiographic changes were believed to be secondary to bacterial infection. Clostridial enteritis was tentatively diagnosed and successfully treated with antibiotics and supportive care. Clostridium tertium infection was hypothesized to have originated from the combination of a contaminated water source, a high carbohydrate diet, and a lack of adequate sanitation. No further clinical problem occurred after implementation of husbandry recommendations.