Background: Mental health services in people with intellectual disabilities (ID) are frequently limited to psychotropic medication (PM), especially antipsychotics. The objectives of this study were to assess the prevalence rates of PM treatment in adults with ID in Germany and to identify factors associated with antipsychotic treatment. Methods: This study is an epidemiological, cross-sectional study. Sampling was realised by a random selection of service-providing institutions, followed by a random selection of adults with ID within these participating institutions. Interviews were conducted with formal and informal carers of n = 197 adults with ID. Data were analysed using descriptive statistics and risk ratios. Results: The 4-week prevalence rate of PM was 53.8%. Antipsychotics were the most frequent PM (43.7%). Polypharmacy and off-label use were common. Antipsychotic treatment is associated with living in a residential home [relative risk (RR) = 2.99], not working in a sheltered workshop (RR = 1.46), autism spectrum disorder (RR = 1.89), a documented psychiatric diagnosis (RR = 1.61), psychiatric symptomatology (RR = 1.44) and (mild) challenging behaviours (RR = 4.58). Conclusions: Further efforts are needed to provide adequate mental health care, specifically to improve PM treatment regarding amount, indication and the consideration of non-psychopharmacological treatment options.