Background: Adult vitamin D deficiency, a public health problem in low-income countries, is correlated with increased mortality. Although health care workers (HCWs) in Ethiopia are educated on minerals counseling on the benefits of vitamin D and its sources is not witnessed since their knowledge is low. The main aim of this study is to explore barriers to good knowledge, positive attitude, and practice of health care providers on adult vitamin D.Methods: An exploratory qualitative study is done in three ecologies covering highland, midland, and lowland. The size of 27 participants was fixed by saturation of ideas. In-depth interviews were conducted among leaders and HCWs of different professional groups in health centers and hospitals. Moderators were Ph.D. holders. An interview guide was developed after reviewing research that was translated into the local language. Interviews were audio-taped, transcribed and, translated. Open Code software was used to code and categorize the data. Thematic analysis is presented using themes and sub-themes.Result: The main barriers are related to health systems, HCWs’ understandings, health facilities, and the educational system. Lack of attention by the health systems’ leadership, missing adult vitamin D as a priority in health programs, and the absence of adult vitamin D management and treatment guidelines are barriers related to the health system. On the other hand, health care professionals believed that the prevalence of adult vitamin D deficiency is insignificant and vitamin D deficiency is only an issue related to children. Besides, the inadequacy of laboratory investigation and limited in-service training are barriers related to health facilities while the absence of studies and the focus of the medical curriculum on child vitamin D are barriers related to the education system. Conclusion: Adult vitamin D deficiency is a neglected public health problem with many barriers related to diagnosis and treatment. Barriers are related to the professionals themselves, their leadership, health facilities, and the education system. The government should give attention to adult vitamin D management and treatment, continuous on the job training, development of guidelines, purchase of laboratory equipment, the inclusion of adult vitamin D in pre-service and, in-service training curriculums.