Medications for opioid use disorder (OUD) and overdose have been available for decades, yet nearly 70% of fatal drug overdoses in the United States are attributed to the opioid receptor agonist fentanyl and its analogs. There is a pressing need for more and better medications that reduce fentanyl use and prevent overdose. A humanized (h) monoclonal antibody (mAb) targeting fentanyl, hHY6-F9, was tested for attenuating intravenous fentanyl self-administration and reversing and preventing fentanyl-induced ventilatory depression in rhesus monkeys. A single administration of hHY6-F9 significantly decreased fentanyl, but not heroin or cocaine, self-administration. In some monkeys, fentanyl self-administration remained decreased for ~ 2 weeks. hHY6-F9 was as effective as 32 µg/kg naloxone in reversing fentanyl-induced ventilatory depression, with a single administration protecting against fentanyl-induced ventilatory depression for 2-3 weeks. Moreover, pharmacokinetic analyses indicate that hHY6-F9 continued to sequester fentanyl in the serum for 2 weeks. This study demonstrates that hHY6-F9 selectively attenuates the positive reinforcing and ventilatory depressant effects of fentanyl, indicating its possible utility for preventing relapse and overdose.