BACKGROUND: Internationally, adult asthma medication adherence rates are low. Studies characterizing variations in barriers by country are lacking. OBJECTIVE: To conduct a scoping review to characterize international variations in barriers to asthma medication adherence among adults. METHODS: MEDLINE, EMBASE, Web of Science (WOS), and CINAHL were searched from inception to February 2017. English-language studies employing qualitative methods (eg, focus groups, interviews) were selected to assess adult patient- and/or caregiver-reported barriers to asthma medication adherence. Two investigators independently identified, extracted data, and collected study characteristics, methodologic approach, and barriers. Barriers were mapped using the Theoretical Domains Framework and findings categorized according to participants' country of residence, countries' gross national income, and the presence of universal health care (World Health Organization definitions). RESULTS: Among 2942 unique abstracts, we reviewed 809 full texts. Among these, we identified 47 studies, conducted in 12 countries, meeting eligibility. Studies included a total of 2614 subjects, predominately female (67%), with the mean age of 19.1 to 70 years. Most commonly reported barriers were beliefs about consequences (eg, medications not needed for asthma control, N = 29, 61.7%) and knowledge (eg, not knowing when to take medication, N = 27, 57.4%); least common was goals (eg, asthma not a priority, N = 1, 2.1%). In 27 studies conducted in countries classified as high income (HIC) with universal health care (UHC), the most reported barrier was participants' beliefs about consequences (N = 17, 63.3%). However, environmental context and resources (N = 12, 66.7%) were more common in HIC without UHC. CONCLUSION: International adherence barriers are diverse and may vary with a country's sociopolitical context. Future adherence interventions should account for trends.