Growing evidence suggests an unexpected association between generalised joint hypermobility (GJH) and several psychiatric conditions, and a shared pathophysiology has been proposed. No previous studies on adult attention-deficit/hyperactivity disorder (ADHD) are available. This study aimed to evaluate the association between adult ADHD and GJH. A total of 431 adults with ADHD and 417 non-ADHD controls were included in this cross-sectional comparative study. GJH was assessed by physical examination following the Beighton scoring system (BSS). Furthermore, musculoskeletal symptoms and skin abnormalities were queried to create a proxy for symptomatic GJH (e.g., Hypermobility spectrum disorders and Ehlers-Danlos syndrome) to differentiate this from non-specified GJH defined by BSS only. Logistic regression examined the influence of ADHD and candidate covariates (age, sex, ethnicity) on GJH and symptomatic GJH, respectively. ADHD was significantly associated with GJH, as defined by the BSS, with adjusted odds ratios of 4.7 (95% confidence interval [CI] 3.0-7.2, p < .005). Likewise, ADHD was significantly associated with symptomatic GJH, as defined by the BSS and additional symptoms, with adjusted odds ratios of 6.9 (CI 95% 4.1-11.9, p < .005). Our results suggest that GJH may represent a marker for an underlying systemic disorder involving both connective tissue and the central nervous system. GJH with additional musculoskeletal symptoms and/or skin abnormalities has a considerable stronger link to adult ADHD than non-specified GJH has, and may need awareness in ADHD management. Future studies should investigate the mechanisms behind this association and how comorbid GJH affects ADHD outcome.