Importance: It is unclear whether increased television (TV) and DVD viewing in early childhood from age 1 year decreases development or whether poor development increases TV/DVD viewing. Objective: To investigate the directional association between TV/DVD screen time and performance on developmental screeners in children aged 1 to 3 years. Design, Setting, and Participants: This longitudinal cohort study analyzed data from 57980 children and mothers from a national birth cohort, the Japan Environment and Children's Study. Data were collected in collaboration with 15 regional centers across Japan. The mothers were recruited between January 2011 and March 2014. Analyses using random intercept, cross-lagged panel models were performed for children aged 1, 2, and 3 years. Of 100303 live births, children with missing developmental screening test scores and screen time data, those with congenital diseases or cerebral palsy, and those diagnosed with an autism spectrum disorder were excluded. Statistical analyses were conducted from October 2022 to July 2023. Exposures: TV and DVD screen time. Main Outcomes and Measures: Child development at ages 1, 2, and 3 years was assessed via the mother's or guardian's report using the Ages and Stages Questionnaire, third edition. Results: Of 57 980 included children, 29 418 (50.7%) were male, and the mean (SD) maternal age at delivery was 31.5 (4.9) years. A negative association between screen time and developmental scores was observed. Increased TV/DVD screen times at age 1 and 2 years were associated with lower developmental scores at age 2 and 3 years, respectively (2 years: β = -0.05, 95% CI, -0.06 to -0.04, 3 years: β = -0.08, 95% CI, -0.09 to -0.06). An obverse association was observed from the Ages and Stages Questionnaires, third edition, score in the communication domain at age 1 and 2 years to subsequent screen time (2 years: γ = -0.03, 95% CI, -0.04 to -0.02, 3 years: γ = -0.06, 95% CI, -0.07 to -0.04). Conclusions and Relevance: In this study, increased TV/DVD screen time from age 1 year negatively affected later development. To reduce the negative consequences of excessive media use, researchers and health care professionals should encourage family media management and recommend social support for parents who tend to rely on the media. © 2023 American Medical Association. All rights reserved.