Background: Open gastrectomy causes severe postoperative pain. Therefore, we investigated the opioid-sparing effect of the ultrasound-guided bilateral erector spinae plane block (ESPB) after open gastrectomy. Methods: Adult patients undergoing open gastrectomy were randomly assigned to either the ESPB group (ESPB + fentanyl based intravenous patient-controlled analgesia [IV-PCA]) or a control group (fentanyl based IV-PCA only). The primary outcome was total fentanyl equivalent consumption during the first 24 hour postoperatively.Secondary outcomes were pain intensities using a numeric rating scale at the postanesthesia care unit (PACU) and at 3, 6, 12, and 24 hour postoperatively, and the amount of fentanyl equivalent consumption during the PACU stay and at 3, 6, and 12 hour postoperatively, and the time to the first request for rescue analgesia. Results: Fifty-eight patients were included in the analysis. There was no significant difference in total fentanyl equivalent consumption during the first 24 hour postoperatively between the two groups (P = 0.471). Pain intensities were not significantly different between the groups except during the PACU stay and 3 hour postoperatively (P < 0.001, for both). Time to the first rescue analgesia in the ward was longer in the ESPB group than the control group (P = 0.045). Conclusions Ultrasound-guided ESPB did not decrease total fentanyl equivalent consumption during the first 24 hour after open gastrectomy. It only reduced postoperative pain intensity until 3 hour postoperatively compared with the control group. Ultrasound-guided single-shot ESPB cannot provide an efficient opioid-sparing effect after open gastrectomy.