Pellagra is a deadly nutritional disease caused by niacin deficiency. Although practically eradicated in developed countries, it still affects vulnerable populations. The diagnosis is based on the presence of characteristic dermatitis in sun-exposed areas, diarrhea, and dementia. We report the case of a woman with a clinical picture of hyperpigmentation and hyperkeratinization in exposed areas of the skin, watery diarrhea, and progressive disorientation with disorganized speech. The anamnesis revealed a poor diet regimen composed almost exclusively of cassava root meals. Alternative diagnosis was excluded and nicotinamide supplementation was introduced with progressive resolution of symptoms until complete recovery. This case report highlights the need to maintain a high index of suspicion in the presence of characteristic symptoms for timely diagnosis of this deadly condition with a simple but dramatic curative treatment.