The study looked into the relationship between senior secondary two (SS2) students' academic self-concept and their mathematics achievement in Bekwarra Local Government Area of Cross River State. This study was directed by three research questions and three matching null hypotheses. A sample of 318 senior secondary two (SS2) students offering Mathematics which comprised two hundred (200) female and one hundred and eighteen (118) male SS 2 students drawn from six (6) secondary schools from a population of 1544 SS2 students offering Mathematics in Bekwarra Local Government Area, Cross River State using Taro Yamane's sample size formula and A simple random selection method was employed, namely by balloting. Two instruments were used for data collection titled; Academic Self-Concept Scale (ASCS) and Mathematics Achievement Test (MAT) used for data collection. Cronbach Alpha (∝) method was used to determine the internal consistency of the Academic Self-Concept Scale (ASCS). The reliability coefficient of the Academic Self-Concept Scale (ASCS) was 0.81. Kuder-Richardson 20 formula method was used to determine the reliability coefficients for the Mathematics Achievement Test (MAT). The reliability coefficients obtained was 0.91 for BAT. The research questions and all null hypotheses were answered and tested using simple linear regression at 0.05 level of significant. Results of the study revealed that: % variation in academic achievement of male students in mathematics was due to their self-concept, and that the academic achievement of male students in mathematics is not significantly correlated with their self-concept. On the other hand, 17% variation in the academic achievement of female students in mathematics was due to their self-concept, and the academic achievement of female students in mathematics is significantly correlated with their self-concept. Finally, students’ self-concept accounted for 23% variation in their academic achievement in Mathematics, and that students’ self-concept significantly relate with their academic Achievement in Mathematics in Bekwarra Local Government Area. The researcher recommended among other things, that parents should help their children have a positive self-concept in mathematics from an early age.