Bullying behaviour refers to repeated negative behaviour displayed by one or more person (s) with the intention of hurting the feeling, personality and power of the victim. The objective of this study therefore was to find out the efficacy of Client-Centred and Rational-Emotive Behaviour Therapies in reducing bullying behaviour among in-school adolescents in Ilorin, Nigeria. The study adopted the quasi-experimental research method using a 3×2 factorial design made up of three (3) row groups (two experimental and one control). Stratified random sampling technique was used to select three secondary schools on the basis of location to prevent experimental contamination. Self-report questionnaire was used to purposively select the participants. The primary dependent variable was bullying behaviour and respondents with high score on bullying items and low scores on victimisation items were selected to participate in the treatment. The findings revealed a significant reduction in the bullying behaviour of the in-school adolescents exposed to experimental treatments; Client-Centred Therapy (CCT) produced significant reduction in the bullying behaviour among the in-school adolescents, and Rational-Emotive Behaviour Therapy (REBT) produced significant reduction in the bullying behaviour of the in-school adolescents. It was recommended that CCT and REBT procedures should be employed in modifying bullying behaviours.