ObjectivesTo explore the association between adolescent multiple risk behaviours (MRBs) and educational attainment.DesignProspective population-based UK birth cohort study.SettingAvon Longitudinal Study of Parents and Children (ALSPAC), a UK birth cohort of children born in 1991–1992.ParticipantsData on some or all MRB measures were available for 5401 ALSPAC participants who attended a clinic at age 15 years and/or completed a detailed questionnaire at age 16 years. Multiple imputation was used to account for missing data.Primary outcome measuresCapped General Certificate of Secondary Education (GCSE) score and odds of attaining five or more GCSE examinations at grades A*–C. Both outcome measures come from the National Pupil Database and were linked to the ALSPAC data.ResultsEngagement in MRB was strongly associated with poorer educational attainment. Each additional risk equated to −6.31 (95% CI −7.03 to −5.58, p<0.001) in capped GCSE score, equivalent to a one grade reduction or reduced odds of attaining five or more A*–C grades of 23% (OR 0.77, 95% CI 0.74 to 0.81, p<0.001). The average cohort member engaged in 3.24 MRB and therefore have an associated reduction in GCSE score equivalent to three and a half grades in one examination, or reduced odds of attaining five or more A*–C grades of 75%.ConclusionEngagement in adolescent MRB is strongly associated with poorer educational attainment at 16 years. Preventing MRB could improve educational attainment and thereby directly and indirectly improve longer-term health.