Postoperative analgesia in patients undergoing spinal fusion surgery is challenging due to the invasiveness of the surgical procedure and the frequent use of opioids preoperatively by many patients. Recently, the erector spinae plane (ESP) block has been introduced in our clinical practice as part of a multimodal pain strategy after posterior lumbar interbody fusion surgery. This is a retrospective case–control study evaluating the analgesic efficacy of the ESP block when added to our standard analgesic regimen for posterior lumbar interbody fusion surgery. Twenty patients who received an erector spinae plane block were compared with 20 controls. The primary endpoint was postoperative pain, measured by the numeric rating scale. Secondary outcome measures were opioid use, postoperative nausea and vomiting, and length of stay. Postoperative pain scores in the PACU were lower in patients who received an erector spinae plane block (p = 0.041). Opioid consumption during surgery and in the PACU was not significantly different. Need for patient-controlled analgesia postoperatively was significantly lower in the group receiving an ESP block (p = 0.010). Length of stay in hospital was reduced from 3.23 days (IQR 1.1) in the control group to 2.74 days (IQR 1.6) in the study group (p = 0.012). Adding an erector spinae plane block to the analgesic regimen for posterior lumbar interbody fusion surgery seemed to reduce postoperative pain and length of hospital stay.