Hypovolemia is a systemic state characterized by severe reduction in the effective blood volume with subsequent tissue hypoperfusion. It may be due to fluid loss, decreased water intake, fluid redistribution, or systemic disease. The present study aimed to evaluate the diagnostic significance of the renal artery resistive index (RRI), caudal vena cava (CVC) diameter, and aorta (Ao) diameter in dogs with hypovolemia. For this purpose, 30 dogs (hypovolemic, n = 15; control, n = 15) were investigated. Clinical variables and hematological findings were investigated for each dog. Pulsed-wave Doppler ultrasound was performed to measure the RRI and diameters of the CVC and Ao. Ultrasound examination revealed a markedly elevated RRI (p < 0.001) and significantly reduced CVC (p < 0.001) and Ao (p < 0.001) diameters in hypovolemic dogs compared to controls, reflecting increased vascular resistance and impaired venous return. Biochemical analysis showed significant increases in blood urea nitrogen (BUN) and BUN:Cr ratio, while Cr remained unchanged. Hematological variables demonstrated limited diagnostic value, with only mild increases in packed cell volume (PCV%). Correlation analysis confirmed a strong positive correlation between RRI and BUN (r = 0.917; p < 0.01), RRI and BUN:Cr ratio (r = 0.664; p < 0.01), and CVC and Ao diameters (r = 0.832; p < 0.05). Receiver operating characteristic (ROC) analysis with area under the curve (AUC) identified RRI (AUC = 0.99), CVC diameter (AUC = 0.93), and Ao diameter (AUC = 0.88) as highly accurate markers of hypovolemia, whereas the CVC:Ao ratio and hematological markers provided poor discrimination. Logistic regression confirmed significant diagnostic value for RRI, CVC diameter, Ao diameter, and BUN, but final multivariate analysis revealed RRI as the sole independent early diagnostic marker (p < 0.001; OR: 196.0; 95% CI: 11.12–34.72). In conclusion, RRI measured by Doppler ultrasound is the most reliable and sensitive early diagnostic marker for hypovolemia in dogs, outperforming conventional biochemical and hematological markers.