Introduction and objective: Obesity, once considered a symbol of high social status, has now become one of the main factors contributing to the development of many diseases that adversely affect both the length and quality of life. The aim of this study was to assess the actual level of knowledge about obesity among primary and secondary school students. Materials and methods: A total of 911 students participated in the study, which was conducted using a proprietary questionnaire. The study was approved by the Bioethics Committee of the Medical University of Silesia. Both participants and their legal guardians gave their informed consent to participate. Results: The most frequently indicated cause of obesity was improper nutrition (879; 96.49%). The Chi2 test showed a statistically significant difference between the number of answers given by boys and girls concerning the influence of hereditary factors on the development of obesity. Over 55% (514; 56.42%) of the respondents assessed their knowledge about obesity as good, but almost 15% (130; 14.27%) claimed that obesity is not a disease. The Wilcoxon test for pairs of observations showed a statistically significant difference between the self-assessed knowledge of the subjects and the scores obtained on the basis of the correctness of the answers given (Z = 21.549, p < 0.001). Conclusions: The knowledge of the surveyed students was generally insufficient, and the results showed that students were overly confident regarding their knowledge. This highlights the need for systematic education on a healthy lifestyle from the earliest school years to help prevent obesity.