The study explored the effectiveness of Rational Emotive Behavior Therapy (REBT) in reducing mathematics anxiety among secondary school students in Nigeria. Sixty Senior Secondary Class One students from schools in Isoko North LGA, Delta State, were randomly assigned to either the REBT intervention group (n = 30; M_age = 15.60 years, SD = 1.04; 14 males, 16 females) or a control group (n = 30; M_age = 15.20 years, SD = 0.93; 16 males, 14 females). Mathematics anxiety levels were evaluated using the Mathematics Anxiety Scale. The experimental group received REBT treatment, while the control group received conventional counseling (general sessions on study habits, exam strategies, anxiety management, and time management). Both interventions spanned six weeks, consisting of six sessions lasting 90 min each. The findings revealed that students who received REBT experienced a significant decline in mathematics anxiety from pretest to posttest, with these improvements sustained at follow-up, in contrast to the control group. The study suggests that mathematics teachers should collaborate closely with school counselors and psychologists to identify and support students facing psychological challenges related to mathematics anxiety.