(1) Background: this work reports the clinical case of a man who underwent a kidney transplant at the age of 19 for idiopathic interstitial nephritis resulting in renal failure. (2) Methods: the patient also had severe obesity and underwent four cycles of a very low-calorie ketogenic diet (VLCKD) for weight loss. (3) Results: the cycles of diet therapy carried out during a six-year period allowed a significant weight loss (from 103.2 kg in 2012 to 87.5 kg in 2018) with lean mass conservation (from 49.9 kg to 52.5 kg) and a reduction of fat mass (from 53.3 kg to 35.0 kg), as assessed with bioimpedance analysis. Throughout the diet cycles, body weight remained stable, except for a weight regain that occurred between the 2nd and 3rd cycle. Renal function was preserved despite the known mild chronic kidney disease (creatininemia ranged from 1.11 mg/dL to 1.27 mg/dL, and GRF from 91 to 71 mL/min/1.73 m2). (4) Conclusions: this report shows that a VLCKD, performed under strict medical supervision, is safe and effective even in a complex clinical picture.