Objectives: Emerging evidence highlights the potential advantages of the Mediterranean diet (MD) in preserving kidney function and slowing chronic kidney disease (CKD) progression. However, interventional studies on the MD are scarce in East Asian populations. This randomized crossover trial aimed to assess the safety and short-term impact of the Mediterranean Proper Optimal Balance (MEDi-POB) diet in Korean patients with stage 3–4 CKD. Methods: Fifty patients with CKD were randomly assigned to two groups, each starting with a different 4-week intervention, followed by a 4-week washout period, followed by a switch to the other 4-week intervention. During the MEDi-POB intervention, participants received home delivery of meals twice daily, 5 days a week, while the control intervention comprised being instructed to follow a conventional diet for patients with CKD. Results: Dietary fat, fiber, and niacin intake were significantly higher following the MEDi-POB diet than after following the control diet. The MEDi-POB diet also yielded slightly increased total CO2 levels, indicating effective management of metabolic acidosis. Conversely, sodium and copper intake were significantly lower with the MEDi-POB diet. Caloric intake increased, but body mass index slightly decreased from baseline after the MEDi-POB diet. Dietary potassium intake showed a non-significant increase, with no significant changes in serum and urine potassium levels. Kidney function remained well preserved following the MEDi-POB diet. Conclusions: The MEDi-POB diet offers significant benefits, addressing undernutrition, promoting balanced fiber and micronutrient intake, and preventing obesity, all while ensuring safety in terms of potassium concentrations and preservation of kidney function.