A low-protein diet (LPD) is recommended to patients with non-dialysis advanced chronic kidney disease (CKD) for delaying renal function decline. However, this approach potentially prevents an adequate calorie and micronutrient intake. We examined the influence of an LPD including a renal-specific oral nutrition supplement (RONS) on the nutrition status of patients with stage 3b–5 CKD. This multicenter, open-label study prospectively enrolled patients over 18 years of age, with an estimated glomerular filtration rate (eGFR) between 10 and 45 mL/min/1.73 m2, serum albumin ≥3.0 g/dL, and body mass index ≤30 kg/m2. All participants implemented the LPD with one serving of RONS daily for 6 months. Daily energy intake, nutrition status, renal function, and quality of life were assessed before and after the intervention. Of 53 enrolled patients, 35 (66.0%) completed the study. We found that RONS use increased patients’ energy intake and maintained their serum albumin, nutritional status, and quality of life. Body weight and handgrip strength increased significantly at 6 months after enrollment (p = 0.0357); eGFR slightly decreased at 3 and 6 months after enrollment, suggesting that patients’ residual renal function was preserved. Our findings support the conclusion that patients with non-dialysis advanced CKD may benefit from additional RONS besides their regular diet. Patients with advanced CKD receiving RONS might achieve better nutrition and delay renal function decline.