Trauma-induced shock is a frequent and life-threatening complication in feline patients. Early detection of tissue hypoperfusion is fundamental for improving clinical outcomes; however, due to species-specific physiological responses in cats, conventional markers like the shock index often fail to accurately reflect shock severity. This prospective clinical study aimed to evaluate the prognostic value of the shock index for predicting mortality in traumatized cats and to analyze the relationship between clinical outcome and whole-blood lactate concentrations, the rectal–interdigital temperature gradient (RISG), and the Animal Trauma Triage (ATT) score. Fifty cats presenting with acute trauma were evaluated upon admission and at subsequent time points. Clinical shock was strictly defined by the presence of at least three markers of poor perfusion, including altered mentation, abnormal heart rate, poor peripheral pulses, and hypotension. Data were analyzed using receiver operating characteristic (ROC) curves and multivariable logistic regression. The Shock Index did not significantly distinguish survivors from nonsurvivors at any time point and showed no significant prognostic performance in ROC analysis (p > 0.05). Whole-blood lactate concentration showed modest prognostic discrimination (AUC = 0.678; 95% CI: 0.508–0.848; p = 0.041), with an optimal cut-off value of ≥4.46 mmol/L. However, the wide confidence interval and its lower limit close to 0.5 indicate considerable uncertainty, and this finding should be considered preliminary. The rectal–interdigital temperature gradient showed the highest prognostic performance (AUC = 0.804; 95% CI: 0.677–0.931; p < 0.001), with an optimal cut-off value of <2.05 °C. In multivariable logistic regression, both RISG (OR = 5.016; 95% CI: 1.783–14.109; p = 0.002) and whole-blood lactate concentration (OR = 0.567; 95% CI: 0.374–0.859; p = 0.007) were independently associated with in-hospital survival. These findings suggest that RISG may provide useful prognostic information, whereas whole-blood lactate concentration may offer complementary but preliminary prognostic information and should be interpreted cautiously. The Shock Index showed limited value for predicting in-hospital outcome in traumatized cats.