To identify independent factors associated with prolonged hospital length of stay (LOS) in elderly patients undergoing first-time elective open posterior lumbar fusion surgery. We retrospectively analyzed the data of 303 elderly patients (age range: 60–86 years) who underwent first-time elective open lumbar posterior fusion surgery at our center from December 2012 to December 2017. Preoperative and perioperative variables were extracted and analyzed for all patients, and multivariate stepwise regression analysis was used to determine the variables affecting the LOS and important predictors of LOS prolongation (P < .001). The mean age of the patients was 67.0 ± 5.5 years, and the mean LOS was 18.5 ± 11.8 days, ranging from 7 to 103 days. Of the total, 166 patients (54.8%) were men and 83 patients (27.4%) had extended LOS. Multiple linear regression analysis determined that age (P < .001), preoperative waiting time ≥7 days (P < .001), pulmonary comorbidities (P = .010), and diabetes (P = .010) were preoperative factors associated with LOS prolongation. Major complications (P = .002), infectious complications (P = .001), multiple surgeries (P < .001), and surgical bleeding (P = .018) were perioperative factors associated with LOS prolongation. Age (P < .001), preoperative waiting time ≥7 days (P < .001), infectious complications (P < .001), and multiple surgeries (P < .001) were important predictors of LOS prolongation. Extended LOS after first-time elective open posterior lumbar fusion surgery in elderly patients is associated with factors including age, preoperative waiting time, infectious complications, and multiple surgeries. Surgeons should recognize and note these relevant factors while taking appropriate precautions to optimize the modifiable factors, thereby reducing the LOS as well as hospitalization costs.