The purpose of this study is to investigate the number sense performance of secondary school students according to grade level, gender and the components of number sense. A descriptive survey design was used to collect data. A total of 576 secondary school students (291 girls and 285 boys) participated in the study. The results revealed that the number sense performance of secondary school students was very low for each grade level. Among the components of number sense, the lowest mean score was observed in multiple representations. Moreover, the number sense performance scores of the students differed significantly in terms of their grade level, though no significant difference was found according to gender. The investigation concluded that the number sense performance of secondary school students is not satisfactory and needs to be improved.