Mu-opioid receptors (MOPRs) are critically involved in the modulation of pain and analgesia, and represent a candidate mechanism for the development of biomarkers of pain conditions and their responses to treatment. To further understand the human implications of genetic variation within the opioid system in pain and opioid-mediated placebo responses, we investigated the association between the functional single-nucleotide polymorphism (SNP) in the μ-opioid receptor gene (OPRM1), A118G, and psychophysical responses, personality traits, and neurotransmitter systems (dopamine (DA), opioid) related to pain and placebo analgesia. OPRM1 G carriers, compared with AA homozygotes, showed an overall reduction of baseline μ-opioid receptor availability in regions implicated in pain and affective regulation. In response to a sustained painful stimulus, we found no effect of A118G on pain-induced endogenous opioid release. Instead, AA homozygotes showed a blunted DA response in the nucleus accumbens (NAc) in response to the pain challenge. After placebo administration, G carriers showed more pronounced mood disturbances and lower placebo-induced μ-opioid system activation in the anterior insula (aINS), the amygdala (AMY), the NAc, the thalamus (THA), and the brainstem, as well as lower levels of DA D2/3 activation in the NAc. At a trait level, G carriers reported higher NEO-Neuroticism scores; a personality trait previously associated with increased pain and lower placebo responses, which were negatively correlated with baseline μ-opioid receptor availability in the aINS and subgenual anterior cingulate cortex (sgACC). Our results demonstrate that the A118G OPRM1 polymorphism contributes to interindividual variations in the function of neurotransmitters responsive to pain (endogenous opioid and dopamine), as well as their regulation through cognitive-emotional influences in the context of therapeutic expectations, the so-called placebo effect. These effects are relevant to human vulnerability to disease processes where these neurotransmitters have a role, such as persistent pain, mood, and substance use disorders, and responses to their treatments.