Kidneys are vital for regulating hydration, erythropoiesis, elimination of toxins, and maintaining electrolyte balance. However, measuring certain parameters in dogs' serum or urine does not clearly distinguish between acute and chronic kidney dysfunction (AKD, CKD). This investigation aimed to approach the diagnostic and prognostic significance of serum cystatin C (Cys-C), a renal function marker, in dogs with AKD and CKD. The current study included 39 dogs total, 10 dogs served as the control group, and 22 dog were diagnosed with AKD and 7 dogs were diagnosed with CKD, with age range (1-13 years) and sex (22 males and 17 females). All dogs underwent clinical and physical examinations, a hematological profile including kidney and hepatic function tests, and urinalysis, as well as renal function marker (Cys-C). Hematology revealed anemia, thrombocytosis and leukocytosis in both AKD and CKD groups, while serum biochemistry revealed a significant increase in alkaline phosphatase (ALKP), cholesterol, glucose, ammonia, blood urea nitrogen (BUN), creatinine (Cr), potassium (K+), phosphorus levels, and Cys-C as well as significant decrease in BUN/Cr and Na⁺/K⁺ ratios in both groups, along with a decrease in calcium value and urine specific gravity in CKD. Urinalysis revealed a significant increase in urinary protein creatinine (UPC) ratio in both groups. Furthermore, Cys-C demonstrated sensitivity and greater specificity than creatinine as a prognostic indicator in AKD and CKD cases. As a result, we can conclude that the serum Cystatin C test holds a significant value as screening diagnostic and prognostic marker for renal dysfunction in dogs. Its results are correlated in different scenarios in dogs' cases, providing crucial information for diagnosing and prognosing various conditions from acute and chronic renal diseases.