This study looked into how students' self-confidence affected their performance in mathematics in South Korean high schools. This study employed an ex-post facto quantitative methodology with a correlational design on a sample size of 288 respondents selected using Yamane formulae. Students and math teachers from Daeyeon High School made up the sample size. To gather quantitative data, a mathematics achievement test and a confidence questionnaire in mathematics were employed. The null hypothesis, according to the Pearson product-moment correlation coefficient computation formula, is that there is no statistically significant correlation between students' self-confidence and mathematical achievement. The study discovered a statistically significant positive correlation (r=723, p=0.000) between math achievement and student confidence. Multiple linear regression analysis revealed that the independent variables in the model account for 70.8% of the variance in the dependent variable, indicating their critical role in high school mathematics achievement for students. Furthermore, it was concluded that there is a significant positive attitude towards students' mathematics achievement when it comes to their self-confidence. The study concluded that since student confidence is directly correlated with mathematical achievement, math teachers should help students solve mathematical problems in order to help them build self-confidence for better achievement