Diabetes mellitus is one of the most common endocrine diseases of cats. Administration of insulin and feeding low-carbohydrate high-protein diets represent the mainstay of treatment. After diagnosis, regular examinations are necessary to monitor glycemia and adjust insulin therapy. Portable blood glucose meters (PBGM) were developed to improve glycemic control in diabetic humans and were validated for use in animals. In the last decade continuous glucose monitoring systems (CGMS) that measure glucose concentrations in the interstitial fluid of the subcutaneous tissue have been developed for humans. A new generation of CGMS allows frequent measurements of interstitial glucose concentrations and instantaneously displays the recorded data on a portable monitor. The aim of our studies was to evaluate the new generation CGMS Guardian REAL-Time® for use in diabetic cats. Accuracy and precision were calculated in vivo and in vitro. The CGMS was clinically reliable and suitable for real-time measurement in cats. Further, performance of the CGMS to generate glucose curves and deciding about treatment was compared to that of a PBGM. Insulin dose recommendations did not differ between glucose curves assessed with CGMS and PBGM. Of note, glucose nadirs were lower with the CGMS, suggesting that the new device detects hypoglycemic periods that may not be identified with the PBGM. Using CGMS is expected to improve glycemic control and ameliorate quality of life in diabetic cats.