Abstract Background Previous studies report vast mental health problems in sexual minority people. Representative national proportion estimates on self-identifying LGB+ persons are missing in Belgium. Lacking data collection regarding sexual orientation in either census or governmental survey data limits our understanding of the true population sizes of different sexual orientation groups and their respective health outcomes. This study assessed the proportion of LGB+ and heterosexual persons in Belgium, LGB+ persons’ self-identification as sexual minority, mental health, and experienced minority stress. Method A representative sample of 4632 individuals drawn from the Belgian National Register completed measures of sexual orientation, subjective minority status, and its importance for their identity as well as a range of mental-health measures. Results LGB+ participants made up 10.02% of the total sample and 52.59% of LGB+ participants self-identified as sexual minority. Most sexual minority participants ...