This study determined the influence of teachers’ classroom effectiveness on students academic performance in public secondary schools in Delta State, Nigeria. It was descriptive in nature and involved 979 teachers, made up of 450 males and 519 females, drawn from 72 out of the total of 361 public secondary schools in the State by stratified random sampling technique. Academic performance records of 50 students per teacher, which is 48,950 students’ scores were also used. Two questionnaires and a rating scale were used to collect data for the study. Cronbach’s alpha value of 0.98 and 0.79 respectively were obtained from the two questionnaires used for the study. Four hypotheses were tested at the 0.05 level of significance using correlation, simple regression, t-test, and single factor analysis of variance. The results showed that effective teachers produced better performing students. However, the observed differences in students’ performance were statistically not significant. This could be due to the influence of student and school environment related factors which were not included in this study. It was concluded that teachers’ effect is not the only determinant on students’ academic achievement.