Introduction. Diet and physical activity are widely recognized in maintaining health and preventing diseases. Currently, dietary supplements are also commonly believed to play a role in health promotion. However, a dietary supplement lacks single uniform definition within the scope of legal or nutritional sciences. Data on the usage of dietary supplements in Europe are still limited. The aim of this study was to assess subjective and objective knowledge of the inhabitants of south-eastern Poland about diet and supplementation in sport. Materials and Methods. The research was conducted in the south-east of Poland between October 2017 and July 2018 during three events organized by the University of Rzeszów. Initially, the study covered 500 people. During the preliminary analysis, 90 people were excluded from the study because they did not meet the previously well-explained rules of conduct during the study or did not meet the eligibility criteria for the tests. Finally, 410 people were qualified for the study. The survey method was a questionnaire with single-answer option. Results. Our study revealed statistically significant subjective and objective level of knowledge about diet and supplementation in terms of sex. Also, a correlation was found between sex and the fact of being involved in physical activity, as well as the subjects' knowledge of diet and supplementation in sport. Conclusion. High prevalence of dietary supplement use was found in our study population, but still significant percentage of the study group demonstrate inaccurate information about these products. Hence, there is an urgent need to provide the community with education and access to scientific and unbiased information.