Background: Early risk assessment in acute pulmonary embolism (PE) remains challenging, particularly in normotensive patients. Lactate may offer incremental prognostic value beyond conventional tools. We investigated the association between arterial lactate and in-hospital mortality in acute PE. Methods: In this retrospective single-center study, 327 adult patients diagnosed with acute PE by computed tomography pulmonary angiography who underwent arterial blood gas analysis within the first six hours of emergency department presentation were included. Patients were categorized according to the occurrence of in-hospital mortality, including 103 (31.5%) non-survivors and 224 (68.5%) survivors, and their demographic, clinical, laboratory, and echocardiographic characteristics were compared accordingly. Results: Arterial lactate levels were significantly higher in non-survivors than survivors [4.1 vs. 1.9 mmol/L; p < 0.001], with a stepwise increase in mortality across lactate categories (4 mmol/L; p < 0.001). In normotensive patients (n = 211), lactate ≥2 mmol/L was associated with higher mortality compared with <2 mmol/L (35.7% vs. 8.7%; OR 5.8, 95% CI 2.7–12.5; p < 0.001). In multivariable logistic regression analysis performed in normotensive patients, arterial lactate level, PESI score, and the presence of cerebrovascular disease were identified as independent predictors of in-hospital mortality, whereas troponin did not retain independent significance. In normotensive patients, lactate showed better discriminative ability than troponin I (AUC 0.718 vs. 0.553). Conclusions: Arterial lactate levels are independently associated with in-hospital mortality in acute PE. Elevated lactate may help identify high-risk patients even in the absence of hypotension and may provide incremental prognostic value beyond existing risk stratification tools. These findings suggest the use of arterial lactate in early risk assessment.