ObjectiveThere is growing interest in the relationship between sedentary behaviour and mental distress among adolescents, but the majority of studies to date have relied on self-reported measures with poor validity. Consequently, current knowledge may be affected by various biases. The aim of this study was to investigate the cross-sectional and longitudinal association between (1) objectively measured sedentary time and (2) self-reported screen time with mental distress among adolescents participating in The Tromsø Study: Fit Futures, in order to see if the association is dependent on mode of measurement of sedentary behaviour.DesignProspective study.SettingSample drawn from upper secondary school students (mean age 16.3 years at baseline) from two municipalities in Northern Norway participating in The Tromsø Study: Fit Futures 1 and 2.Participants686 adolescents (54.5% female), with complete self-reported and accelerometer data after multiple imputation.Primary outcome measuresMental distress assessed via the Hopkins Symptom Checklist-10 (HSCL-10).ResultsMinutes in sedentary behaviour measured by accelerometer showed no significant relationship with mental distress in neither crude, partly adjusted nor multiple adjusted hierarchic linear regression analyses. Self-reported screen time was positively associated with mental distress in all analyses (multiple adjusted, B=0.038, p=0.008, 95% CI 0.010 to 0.066). However, the effect was small.ConclusionsSelf-reported screen time was associated with slightly elevated mental distress 2 years later, whereas objectively measured minutes in sedentary behaviour was not, indicating a discrepancy in the results depending on measurement methods.