Abstract Background Arterial stiffness and carotid intima-media thickness (cIMT) progression have been associated with cardiovascular morbidity and mortality in adulthood. Arterial stiffness is a strong risk factor for elevated blood pressure and hypertension, and insulin resistance in adolescents and young adults. It remains unclear whether cumulative sedentary time independently worsens arterial health in the paediatric population. Purpose To investigate the longitudinal association of cumulative sedentary time from childhood through young adulthood with arterial stiffness and cIMT changes during post-pubertal growth. Methods From the Avon Longitudinal Study of Parents and Children (ALSPAC), UK birth cohort, 1399 children aged 11 years who had at least two follow-up time-points accelerometer-measured sedentary time over 13 years follow-up, and complete cardiac structural measures at age 17 years clinic visit were included. Sedentary time was assessed with ActiGraph accelerometer worn for 4-7 days at the 11-, 15-, and 24-year clinic visits and sex-categorized in tertiles as low (reference), moderate, and high. Carotid femoral pulse wave velocity (cfPWV), a measure of arterial stiffness, and cIMT were assessed with Vicorder and ultrasound, respectively at ages 17 and 24 years. Multivariable adjusted associations were examined using generalized linear mixed-effect models and adjusted for sex, and time-varying covariates measured at both baseline and follow-up such as age, insulin, high-sensitivity C-reactive protein, heart rate, systolic blood pressure, glucose, fat mass, lean mass, smoking status, family history of hypertension/diabetes/high cholesterol/vascular disease, socioeconomic status, triglyceride, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, light physical activity, and moderate to vigorous physical activity, Results Among 1339 participants (mean [SD] age, 11.75 [0.24] years; [54.6% females]) males spent on average 358, 461, and 536 minutes/day sedentary at ages 11, 15, and 24 years, respectively. Females spent 369, 484, and 525 minutes/day sedentary at ages 11, 15, and 24 years respectively. cfPWV was significantly higher among males than females at ages 17 and 24 years. cIMT was higher in males than females at 17 years but not at age 24 years. In a fully adjusted model, the highest tertile of sedentary time from ages 11 – 24 years was cumulatively associated with progressively increased changes in cfPWV (effect estimate 0.009m/s [CI 0.006 – 0.012] p<0.001) from ages 17 – 24 years in the total cohort and females (0.047m/s [CI 0.005 – 0.090] p=0.030), but not in males. There was no statistically significant association between cumulative sedentary time and changes in cIMT in the total cohort, males, and females. Conclusion Cumulative increase in sedentary time during growth from childhood through young adulthood was independently associated with progressively worsening arterial stiffness in youth.