Background: To contain the COVID-19 pandemic, governments worldwide restricted social and physical contact by issuing lockdown and social-distancing measures. Yet, lockdown measures may induce mental health problems and increase the occurrence of domestic violence (DV). We examine mental health and DV in lesbian, gay, bisexual, pansexual and asexual (LGB+) persons under lockdown. Methods: An online self-report questionnaire on relationships, stress and aggression was administered to a non-probabilistic sample of participants living in Belgium. Participants were sampled through national media, social media, and snowballing procedures. Occurrence of DV including psychological, physical and sexual violence, stress, alcohol and drug use, suicidal ideation, suicide attempt, self-harming behaviour, and help-seeking behaviour in LGB+ persons during the first four to six weeks of the hygiene and lockdown measures in Belgium were assessed. Results: 383 LGB+ participants were included in the analysis. In addition to high levels of stress, alcohol and drug abuse, suicidal ideation and self-harming behaviour, a third of LGB+ participants reported at least one incident of DV under lockdown. Conclusion: LGB+ persons have been exposed to DV and experienced lower mental health and wellbeing during the lockdown related to the COVID-19 pandemic. These findings highlight the possible need for public health measures and sociocultural changes preventing DV and improving mental health during lockdown in LGB+ persons.