The aim of this study was to describe the clinical course of a morbidly obese patient who underwent Roux-en-Y gastric bypass (RYGB) surgery and, in the late postoperative period, presented the expected loss of weight, but also presented severe protein malnutrition (PM). A patient with morbid obesity, who in March 2012, presented PM (serum albumin = 2.4 g/dL) 2 y after the completion of RYGB surgery (loss of 52.7% of usual body weight). During the hospitalization, the patient received partial volumes of commercial semi-elemental, high-protein, low-fat diet by tube feeding with gastric positioning, associated with an oral low-fat, low-sodium, and bland-consistency diet. The patient presented a temporary clinical improvement, however, outpatient monitoring identified the need for subsequent hospitalizations due to the recurrence of severe hypoalbuminemia (e.g., 1.39 g/dL), anasarca (increase of 15 kg in 79 d), and normocytic and normochromic anemia (e.g., hemoglobin 9.2 g/dL). In July 2013 the RYGB partial reversal technique was performed with a reduction of 100 cm in the Roux-en-Y arm. Seventy days after surgery, the patient was asymptomatic (albumin 3.7 g/dL), however, she presented rapid and progressive recovery of the body weight (increase of 10.3 kg in 60 d, without edema). The effective treatment of morbid obesity is still a major challenge in clinical practice. Restrictive, malabsorptive bariatric techniques are associated with nutritional deficiencies. Severe PM is rarely reported as a late postoperative complication of RYGB, however, due to the serious consequences associated with this, it requires early diagnosis and treatment.