Low back pain (LBP) is a common health problem already in adolescence. Physical activity has been suggested as a risk factor for LBP in adolescents, but the current evidence is conflicting. This study examined the association of physical activity and amount of sitting with LBP. The study population consisted of 5999 boy and girl members of the Northern Finland 1986 birth cohort who responded to mailed questions at the age of 15–16 years. LBP during the past 6 months was classified as “no LBP,”“reporting LBP” (not seeking medical help), or “consultation for LBP.” Odds ratios and 95% confidence intervals obtained by multinomial logistic regression were adjusted for smoking and body mass index. Being physically very active (more than 6 h of brisk physical activity per week) was associated with increased prevalence of “consultation for LBP” in both sexes, and with “reporting LBP” in girls, compared with being moderately active (2–3 h of brisk physical activity per week). High amount of sitting associated with “consultation for LBP” and “reporting LBP” in girls, but not in boys. We conclude that very active participation in physical activities in both sexes and a high amount of sitting in girls are related to self‐reported LBP.