The study looked into how students’ mathematics achievement was impacted by the quality, experience, and subject-matter expertise of teachers. The correlation method's ex-post-factor design was used in this investigation. All of the public secondary schools in Delta State, Nigeria's Okpe Local Government Area made up the study's population. The sample size consisted of three hundred (300) teachers selected from thirteen (13) schools. Two tools were employed to gather the data. These were the third-term mathematics results for the 2023–2024 school year and the Teacher Quality, Experience, and Content Knowledge Questionnaire. Both content and face validation were applied to the instrument. The dependability of the questionnaire instrument was tested using Cronbach's alpha, which produced a reliability coefficient of 0.71. Since the scores were gotten from the original score sheet, the achievement outcomes did not need to be tested for reliability. Coefficient of determination was used to answer the research questions, and linear regression was used to test the hypotheses at the 0.05 level of significance. The results showed that students' mathematics performance in Delta State's Okpe Local Government Area was positively correlated with the calibre, experience, and subject-matter expertise of the teachers. The results also demonstrated a statistically significant correlation between learners' mathematical achievement and the calibre and expertise of teachers. Therefore, it was suggested that school heads start mentorship programs that pair seasoned teachers with newcomers, prioritise ongoing professional development programs that improve teachers' quality and subject matter knowledge, and promote the use of evidence-based instructional methods that have been shown to increase students’ engagement and learning.