Background: This study aimed to investigate the electrocardiographic risk factors associated with mortality in elderly patients diagnosed with pulmonary thromboembolism (PTE). Methods: This multicenter, retrospective study analyzed electrocardiograms (ECGs) of patients aged 80 years and older at the time of their initial PTE diagnosis. Additionally, patient outcomes were assessed to determine whether death occurred due to PTE or related complications within a follow-up period of 180 days. The relationship between electrocardiographic findings and 30-day and 180-day mortality was examined. Results: A total of 241 patients were included, with a mean age of 83.5 ± 2.6 years; 143 (59.3%) were female. The most common electrocardiographic abnormality was sinus tachycardia, observed in 73.4% of patients. Mortality was recorded in 64 patients (26.5%) within 30 days and 88 patients (36.5%) within 180 days. The strongest predictor of 30-day mortality was ST segment (ST) depression in leads V4–6 (odds ratio (OR) 14.5; 95% confidence interval (CI) 5.1–41.0), while the most significant predictor of 180-day mortality was ST elevation in leads V1, augmented Vector Right (aVR) lead, or III (OR 15.8; 95% CI: 6.6–37.7). Conclusions: Pulmonary thromboembolism in octogenarians is associated with high mortality. Electrocardiographic evaluation at the time of diagnosis is crucial, as specific ECG abnormalities may serve as important prognostic indicators for 30-day and 180-day mortality.