BACKGROUND: Having electronic media in the bedroom is cross-sectionally associated with greater screen-time in children, but few longitudinal studies exist. The aim of this study was to describe longitudinal patterns of ownership and examine cross-sectional and longitudinal associations of bedroom media with children’s sedentary behaviour. METHODS: Data are from the Sport, Physical activity and Eating behaviour: Environmental Determinants in Young people (SPEEDY) study, collected at 3 time-points: baseline (2007, T₀; age 10.3 ± 0.3 years), 1-year (T₁y) and 4-year (T₄y) follow-up. For each assessment, 1512 (44.9% male), 715 (41.0% male), and 319 (48.3% male) participants provided valid accelerometer data. Outcome variables were accelerometer-assessed sedentary time and self-reported screen-time. The presence of a television or computer in the bedroom was self-reported by participants and a combined bedroom media score calculated as the sum of such items. Cross-sectional and longitudinal associations between bedroom media and each outcome were examined using multi-level linear regression. RESULTS: Bedroom TV ownership fell from 70.9% at T₀ to 42.5% at T₄y. Having a TV in the bedroom (beta; 95% CI*100, T₀: -1.17; -1.88, -0.46. T₁y: -1.68; -2.67, -0.70) and combined bedroom media (T₀: -0.76; -1.26, -0.27. T₁y: -0.79; -1.51, -0.07) were negatively associated with objectively measured weekly sedentary time at T₀ and T₁y. Having a computer in the bedroom (beta; 95% CI, T₀: 0.15; 0.02, 0.29. T₄y: 0.35; 0.10, 0.60) and combined bedroom media (T₀: 0.09: 0.01, 0.18. T₄y: 0.20; 0.05, 0.34) were positively associated with screen-time at T₀ and T₄y. Relative to participants without a computer throughout the study, children that had a computer in their bedroom at T₀ but not at T₄y (beta; 95% CI for change in screen-time: -8.02; -12.75, -3.29) reported smaller increases in screen-time. CONCLUSIONS: The bedroom media environment changes with age and exhibits a complex relationship with children’s sedentary behaviour. Modifying children’s bedroom media environment may impact upon screen-time but appears unlikely to influence overall sedentary time.