English language plays the vital roles of being the medium of instruction in Nigerian schools from primary to tertiary level. The knowledge of the contents of school subjects is transferred to the students at all levels of education via English language medium. It therefore follows that how well students would fare in academic attainment depends largely on their level of proficiency in English language which is the medium of instruction. This paper therefore examined the extent to which Nigerian secondary school students’ proficiency in English predicted their overall academic achievement in Oyo and Osun States of Nigeria. From the population of students in the two states, 400 students were proportionately sampled from eight randomly sampled secondary schools. Two Research questions guided the study in which two Research Instruments namely, English Language Proficiency Test (ELPT) was used. The performance of the respondents in the ELPT was correlated with the students’ annual scores in cores school subjects – Mathematics, Biology and English using Pearson Product Moment Correlation and Linear Regression Analysis at .05 level. The results showed that English language proficiency of the students has a significant positive relationship with their overall academic achievement and that there is a significant impact of English language proficiency on students overall academic achievement. Based on these findings, it is recommended that efforts should be geared towards making the Nigerian students proficient in English as a way of improving their academic performance.