Prior evidence has supported the use of local intraoperative epidural steroids in lumbar discectomy for improvements in postoperative pain and outcomes. However, currently there is a paucity of data regarding the efficacy of local epidural steroids in spinal fusion procedures. The present investigation aimed to evaluate the impact of local epidural administration of triamcinolone acetonide-soaked Gelfoam on postoperative pain and patient-reported outcomes in patients undergoing instrumented posterolateral lumbar spinal fusion. In this randomized, double-blind, placebo-controlled trial, patients were randomly divided into two groups (treatment and control). Patients in the treatment group received a Gelfoam carrier soaked in 1 ml of triamcinolone acetonide (40 mg), which was placed over the nerve roots in the epidural space before the closure. Patients in the control group received a Gelfoam carrier soaked in normal saline in a similar fashion to the treatment group. Patients were followed up during their hospital stay and at 4 and 12 weeks postoperatively. The primary outcome measure was early postoperative visual analog scale (VAS) scores for pain both at rest and with movement. A total of 100 patients were recruited in this study and were randomly allocated to the treatment or control group. No significant difference was found in baseline demographic, clinical, and surgical characteristics between the two groups. Postoperative VAS scores for pain both at rest and with movement were comparable between the treatment and control groups. Cumulative morphine consumption, length of hospital stay, and incidence of postoperative complications such as surgical site infection were also similar between the two groups. There was no significant difference in patient-reported outcomes including VAS scores for back and leg pain as well as the Oswestry Disability Index at 4 and 12 weeks postoperatively. The proportion of patients who achieved a minimum clinically important difference for patient-reported outcomes were also similar between the two groups. In contrast to the existing literature on the beneficial use of local intraoperative epidural steroids in conventional lumbar discectomy, the present study did not demonstrate such significant efficacy for the use of local epidural steroids in instrumented posterolateral lumbar spinal fusion. However, there is still a lack of evidence in this regard and further high-quality clinical trials are required to evaluate the efficacy of local epidural steroids in this group of patients.