The present study investigated gender differences in adolescents' academic motivation and classroom behaviour and gender differences in the extent to which motivation was associated with, and predicted, classroom behaviour. Seven hundred and fifty students (384 boys and 366 girls) aged 11--16 (M age?=?14.0, 1.59 SD) completed a questionnaire examining academic motivation and teachers completed assessments of their classroom behaviour. Girls generally reported higher levels of academic motivation, whilst teacher reports of behaviour were poorer for boys. Interestingly, boys' reported levels of academic motivation were significantly more closely associated with teacher reports of their classroom behaviour. Furthermore, cognitive aspects of boys' motivation were better predictors of their classroom behaviour than behavioural aspects. On the other hand, behavioural aspects of girls' motivation were better predictors of their behaviour. Implications for understanding the relationship between motivation and behaviour among adolescent boys and girls are discussed, in addition to interventions aimed at improving adolescents' classroom behaviour.