AIM: To examine the relationships of physical activity (PA) and sedentary behaviour (SB) with suicidal thoughts and behaviour among adolescents in low- and middle-income countries (LMICs). METHODS: Global School-based Student Health Survey data from 206,357 students (14.6±1.18 years; 51% female) in 52 LMICs were used. Students reported on suicidal ideation, suicide planning, suicide attempts, PA, SB, and socio-demographic characteristics. Multilevel mixed-effects generalised linear modeling was used to examine the associations. RESULTS: High SB (≥3 hrs/day) was independently associated with higher odds of suicidal ideation, planning, and attempts for both male and female adolescents. Insufficient PA (<60 mins/day) was not associated with higher odds of ideation for either sex; however, was associated with planning and attempts for male adolescents. The combination of insufficient PA and high SB, compared to sufficient PA and low SB, was associated with higher odds of suicide ideation and planning for both male and female adolescents, and suicide attempts for male adolescents. CONCLUSION: High SB may be an indicator of suicidal vulnerability among adolescents in LMICs. Low PA may be a more important risk for suicidal thoughts and behaviours among male, than female, adolescents. Promoting active lifestyle should be integrated into suicide prevention programmes in resource-poor settings.