Patients with mental illness are at an increased risk of COVID-19 infection, morbidity, and mortality, and prioritisation of this group for COVID-19 vaccination programmes has therefore been suggested. Vaccine uptake may, however, be compromised by vaccine hesitancy amongst patients with mental illness, posing a critical public health issue. We conducted two surveys to provide weighted estimates of vaccine willingness amongst patients with mental illness and the general population of Denmark. Vaccine willingness was high in both groups, but slightly lower amongst patients with mental illness (84.8%), compared with the general population (89.5%) (p < .001). Based on these findings, vaccine hesitancy does not appear to be a major barrier for vaccine uptake amongst patients with mental illness in Denmark, but may be so in other countries with lower general vaccine willingness. Replication of the present study in other countries is strongly warranted.