Background Research into the impact of social relationships on childhood and adolescent health and wellbeing has been largely limited to children’s relationships with other humans, while studies into the impact of pet ownership are sparse and have generally not adjusted for potential confounders. This study aimed to investigate the association between pet ownership and a range of developmental outcomes in childhood and adolescence. Methods Data were self-reports and direct assessments of approx. 14,000 children from the Avon Longitudinal Study of Parents and Children (ALSPAC). Multivariable regression models adjusting for confounding factors examined associations between developmental outcome measures (emotional health, behavioural development, cognitive development, language development, educational attainment) and concurrent pet ownership, including species, and also longitudinal pet ownership history and pet-interaction where possible. Analyses model numbers using multiple imputation varied from n = 393–8963. Results In cross-sectional analyses, owning a dog (b = 0.24, [0.06–0.41], p = .004) and owning other/miscellaneous pets (b = 0.18, [0.03–0.33], p = .021) at age 3 were associated with higher prosocial behaviour score. Owning a pet was associated with a higher non-verbal communication score at age 2 (cross-sectional, b = 0.18, [0.04–0.32], p = .014), and a higher language development score at age 5 (cross-sectional, b = 1.01, [0.18–1.83], p = .017). However, pet ownership was associated with lower educational attainment across a number of academic subjects and timepoints, in both cross-sectional and longitudinal analyses. It was also cross-sectionally linked to hyperactivity at age 3 and conduct problems at age 3 and 11. Furthermore, at age 8, cross-sectional analysis showed that children who owned any pets (OR [95% CI]: 0.85 [0.73–0.98], p= ·026) or cats (0.83, [0.73–0.95], p= ·006) had lower odds of high self-esteem (scholastic competence). Conclusions Usin