Children with deviant behaviours display difficulties adjusting to the classroom environment and also show social skill deficits, and low academic performance. This paper examined the psychosocial problems and academic achievement of deviant pupils in basic schools in Oyo State, Nigeria. This study was a descriptive survey research method of the correlational type. Three purposes and three research questions were raised and two hypotheses were tested. The sample for this study comprised 171 deviant pupils in public basic six in the two selected senatorial districts. Twenty Basic Schools were selected from the four Local Government Areas using proportionate random sampling technique. Three instruments were used for this study: a checklist on the types of deviant behaviour, a researcher-designed questionnaire titled "Psychosocial Scale (PS)" and an Academic Achievement Test (AAT). The findings of the study were that: the most prevalent deviant behaviour among deviant pupils was coming late to school. There was no significant difference in the psychosocial problem and academic achievement of male and female deviant pupils in Basic Schools in Oyo state. It was recommended that parents should ensure that their children leave home as early as possible in order not to get late to school and also, punishment should be melted out to late comers in order to reduce the rate at which pupils come late to school, teachers should find out why pupils come late to school either by raising during Parent-Teachers' Association meeting and they dialogue and discuss the way out of it with the parents. Teacher should have positive attitude to deviant pupils because it helps to them have positive feelings about themselves. Deviant behaviours among pupils, if not checked, can lead to societal vices, because of this, seminars should be organized for teachers to enhance their knowledge on effective behaviour modification techniques. The schools should provide record book to keep details of cases and to monitor pupils' progressive behaviour changes and learning needs in order to know whether the rate of deviant behaviour is reducing or increasing.