This study investigates the impact of systematic desensitization (SD) on mathematics anxiety and achievement among Nigerian secondary school students. Employing a quasi-experimental design with a pretest and post-test nonequivalent control group design, we selected 120 students (58 boys, 62 girls) from a pool of 3390 senior class one students using a multi-stage sampling procedure. Assessment tools included the Mathematics Achievement Test and Mathematics Anxiety Scale. The six-week intervention demonstrated significant results with moderate effect sizes, revealing that SD significantly reduced mathematics anxiety (F[1,115] = 86.66, p < .001) and enhanced academic achievement (F[1,115] = 92.458, p < .001). Gender showed no substantial influence on mathematics anxiety (F[1,115] = .365, p = .547) or achievement under SD treatment (F[1,115] = .006, p = .937). This study underscores the effectiveness of SD in reducing mathematics anxiety and improving mathematics performance among Nigerian students.