This systematic review and meta-analysis examines the dose-response relationship between sedentary behavior (total daily sedentary time and TV viewing time) and all-cause, cardiovascular disease (CVD), and cancer mortality, as well as incident type 2 diabetes (T2D), adjusted for physical activity (PA). The study included 34 prospective studies with 1,331,468 unique participants. The results showed that total sedentary behavior was associated with increased risk of all-cause and CVD mortality, with a threshold of 6-8 hours/day for all-cause mortality and 6-8 hours/day for CVD mortality. TV viewing time was also associated with increased risk, with a threshold of 3-4 hours/day for all-cause and CVD mortality. The association with cancer mortality was linear, while the association with T2D was linear for TV viewing time but not for total sedentary time. PA adjustment attenuated the effect estimates, particularly for TV viewing time. The population attributable fraction (PAF) for T2D was estimated to be 29% in England, while for all-cause and CVD mortality, it was 8% and 5%, respectively. The findings suggest that reducing sedentary behavior, especially TV viewing, could have significant public health benefits.This systematic review and meta-analysis examines the dose-response relationship between sedentary behavior (total daily sedentary time and TV viewing time) and all-cause, cardiovascular disease (CVD), and cancer mortality, as well as incident type 2 diabetes (T2D), adjusted for physical activity (PA). The study included 34 prospective studies with 1,331,468 unique participants. The results showed that total sedentary behavior was associated with increased risk of all-cause and CVD mortality, with a threshold of 6-8 hours/day for all-cause mortality and 6-8 hours/day for CVD mortality. TV viewing time was also associated with increased risk, with a threshold of 3-4 hours/day for all-cause and CVD mortality. The association with cancer mortality was linear, while the association with T2D was linear for TV viewing time but not for total sedentary time. PA adjustment attenuated the effect estimates, particularly for TV viewing time. The population attributable fraction (PAF) for T2D was estimated to be 29% in England, while for all-cause and CVD mortality, it was 8% and 5%, respectively. The findings suggest that reducing sedentary behavior, especially TV viewing, could have significant public health benefits.