BACKGROUND: Previous meta-analyses may have overestimated the prevalence of attention-deficit hyperactivity disorder (ADHD) in prisoners by including data from selected samples. AIMS: To estimate the prevalence of ADHD in unselected samples of adults in prison and examine potential sources of heterogeneity by meta-regression and subgroup analyses. METHODS: We assessed all studies included in a 2018 systematic review for eligibility and updated the literature search to include studies published up to September 2023. RESULTS: A total of 11 studies reporting robust diagnostic data on ADHD in 3919 unselected adults in prison were included. In meta-regression, ADHD prevalence did not differ between men and women but it was significantly higher in studies using a two-phase design. In random sampling studies, the pooled prevalence of ADHD was 8.3% (95% CI: 3.8-12.8) which was further halved after the removal of an outlier. CONCLUSION: One in 12 adults in prison have been diagnosed with ADHD. Our findings highlight the importance of using clear and consistent inclusion criteria in meta-analyses of prevalence.