Noticing the inconsistency in previous studies of gender differences in learning physics, it was determined to check whether a gender gap exists in a relatively large sample of competitors (N = 12,966) from Serbia. The aim of the research presented here was to analyze the achievements of male and female lower secondary school students, competitors in physics, at all rounds of the physics national competitions in Serbia. The population of respondents consisted of 3,392 competitors in the school year 2015/2016, 4404 in 2016/2017, and 5170 in 2017/2018. Participants were grouped by gender. The results of the analyses, based on a t-test with a confidence level of 95% and a = 0.05, support the hypothesis that there is a statistically significant gender gap in favor of male participants at municipal and district rounds of competitions. However, the obtained results show a lack of difference in achievements at the state competition level. In general, at each subsequent level of competition, the gender gap decreases. In addition to the observed differences at the three levels of competition, the similarities in the achievements of male and female students on certain problems were also analyzed.