Objective <p>To identify the determinants of 28-day perioperative mortality and postoperative length of hospital stay among geriatric patients.</p> Design and setting <p>A prospective, two-center follow-up study conducted at two tertiary referral hospitals in Ethiopia.</p> Participants <p>A total of 1014 consecutive geriatric patients who underwent surgery between January 2019 and January 2022 were included. </p> Results <p>This study documented a 28-day perioperative mortality rate of 2.86% (95% CI: 1.9–4.0) and a median postoperative length of hospital stay of six days. Multivariable analyses identified independent determinants for each outcome. For 28-day perioperative mortality, Cox proportional hazards regression revealed significant associations with the presence of comorbidities (aHR = 3.10) and the use of general anesthesia (aHR = 6.12). Determinants of extended postoperative length of hospital stay, evaluated using negative binomial regression, included the following: comorbidities (aIRR= 1.31), emergency surgery (aIRR = 1.28), and higher American Society of Anesthesiologists (ASA) physical status classification (ASA II: aIRR = 1.31; ASA ≥ III: aIRR = 1.38), each additional 60 minutes of anesthesia duration (aIRR = 1.15), and intraoperative blood loss (&gt;500 mL; aIRR = 1.27). Conversely, each 1 g/dL increase in preoperative hemoglobin was found to be protective (aIRR = 0.97). </p> Conclusion <p>This study identifies key modifiable determinants of adverse postoperative outcomes in geriatric surgical patients. The 28-day perioperative mortality was strongly associated with general anesthesia and preexisting comorbidities. The postoperative length of hospital stay was influenced by a broader set of variables, including patient factors, surgical urgency, and procedural complexity. These findings highlight critical targets for quality improvement in low-resource settings. </p>