Introduction An increasing number of clinical and epidemiological studies suggest a possible association between attention-deficit/hyperactivity disorder (ADHD) and obesity/overweight. However, overall evidence is mixed. Given the public health relevance of ADHD and obesity/overweight, understanding whether and to what extent they are associated is paramount to plan intervention and prevention strategies. We describe the protocol of a systematic review and meta-analysis aimed at assessing the prevalence of obesity/overweight in individuals with ADHD versus those without ADHD. Methods and analysis We will include studies of any design (except case reports or case series) comparing the prevalence of obesity and/or overweight in children or adults with and without ADHD (or hyperkinetic disorder). We will search an extensive number of databases including PubMed, Ovid databases, Web of Knowledge and Thomson-Reuters databases, ERIC and CINAHL. No restrictions of language will be applied. We will also contact experts in the field for possible unpublished or in press data. Primary and additional outcomes will be the prevalence of obesity and overweight, respectively. We will combine ORs using random-effects models in STATA V.12.0. The quality of the study will be assessed primarily using the Newcastle-Ottawa Scale. Subgroup meta-analyses will be conducted according to participants’ age (children vs adults) and study setting (clinical vs general population). We will explore the feasibility of conducting meta-regression analyses to assess the moderating effect of age, gender, socioeconomic status, study setting, geographic location of the study (low-income, middle-income countries vs high-income countries), definition of obesity, method to assess ADHD, psychiatric comorbidities and medication status. Ethics and dissemination No ethical issues are foreseen. The results will be published in a peer-reviewed journal and presented at national and international conferences of psychiatry, psychology, obesity and paediatrics. Registration PROSPERO-National Institute of Health Research (NIHR) Prospective Register of Systematic Reviews (CRD42013006410).