Self-administration of addictive substances like heroin can couple the rewarding/euphoric effects of the drug with drug-associated cues, and opioid cue reactivity contributes to relapse vulnerability in abstinent individuals recovering from an opioid use disorder (OUD). Opioids are reported to alter the intrinsic excitability of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a key brain reward region linked to drug seeking, but how opioids alter NAc MSN neuronal excitability and the impact of altered MSN excitability on relapse-like opioid seeking remain unclear. Here, we discovered that self-administered, but not experimenter-administered, heroin reduced NAc protein levels of the voltage-gated sodium channel auxiliary subunit, SCN1b, in male and female rats. Viral-mediated reduction of NAc SCN1b increased the intrinsic excitability of MSNs, but without altering glutamatergic and GABAergic synaptic transmission. While reducing NAc SCN1b levels had no effect on acquisition of heroin self-administration or extinction learning, we observed a significant increase in cue-reinstated heroin seeking, suggesting that NAc SCN1b normally limits cue-reinstated heroin seeking. We also observed that NAc SCN1b protein levels returned to baseline following heroin self-administration, home-cage abstinence, and extinction training, suggesting that the noted reduction of NAc SCN1b during acquisition of heroin self-administration likely enhances MSN excitability and the strength of heroin-cue associations formed during active heroin use. As such, enhancing NAc SCN1b function might mitigate opioid cue reactivity and a return to active drug use in individuals suffering from OUD.