Test anxiety is one of the most pervasive emotional challenges facing secondary school students, yet its impact on academic achievement in the Nigerian context remains underexplored. This study examined the effect of test anxiety on academic achievement among senior secondary school students. A descriptive ex-post facto design was adopted, with 180 students selected through stratified random sampling from two public secondary schools in Rivers State, Nigeria. Data were collected using the Test Anxiety Inventory (TAI) and students’ academic records from three core subjects. Pearson product-moment correlation and independent samples t-test were used for analysis. Findings revealed a significant negative relationship between test anxiety and academic achievement, r(178) = –.55, p < .001. Students with high test anxiety performed significantly lower (M = 58.30, SD = 7.85) than those with low test anxiety (M = 66.60, SD = 8.10), t(178) = –7.84, p < .001. The study concludes that test anxiety is a significant barrier to academic success among Nigerian secondary school students. Recommendations include the integration of anxiety management programmes into school counselling services and the training of teachers to create low-stress assessment environments.