This study investigated the influence of personality traits on the academic achievement of the Secondary School at Harari Regional State, Eastern Ethiopia. The underachievement has assumed a worrisome dimension in the secondary school educational system. This study used the explanatory sequential mixed method design using a standardized instrument as a primary data-gathering tool. The study's respondents were four Secondary School students at Harari Regional State. The stratified random sampling technique was used to select 347 respondents from the randomly selected secondary schools in the study area. The findings of the study found that conscientiousness r(347) =0.46**, was highly statistically significantly positively related to academic achievement, whereas neuroticism r(347) =-0.39**) was highly statistically significantly negatively related to academic achievement. In this study, results from regression analysis showed that conscientiousness was the good and strongest individual contributor or predictor of academic achievement. In conclusion, personality traits significantly affect students’ academic achievement. It was recommended that teachers, parents, and students be educated on the knowledge that personality traits play a major role in how one can react to issues academic activities, and performance in school.