Background: The Group Climate Inventory (GCI) was tested for measurement invariance across 332 adults with and 225 adults without mild intellectual disabilities in Dutch forensic treatment, and for latent mean differences on its "Support", "Growth", "Repression", and "Atmosphere" subscales. Method: Multigroup confirmatory factor analysis was used to evaluate the configural, threshold, and loading and threshold invariance of the GCI across both groups, and to compare group latent means on each subscale. Results: Measurement invariance was found across groups. Latent mean group comparisons showed small but significant differences reflected in lower scores on Support and Atmosphere in the group with mild intellectual disabilities. Conclusion: The GCI allows meaningful comparisons between clients with and without mild intellectual disabilities in secure facilities. Results from the between-group comparisons suggest that consideration should be given as to whether, and why, the support and atmosphere perceptions of clients with mild intellectual disabilities might be less good.