Context: The effect of both birth order and number of siblings on overweight and/or obesity has not been determined. Birth order and sibsize have been mathematically coupled to overweight and/or obesity, but thus far their respective effects have been estimated separately. Objective: The aim of this systematic review was to evaluate the effects of both birth order and number of siblings on the risk of overweight/obesity. Data Sources: The electronic databases MEDLINE, Social Science, SocINDEX, PsycINFO, CINAHL Plus, and Academic Search Complete were searched systematically. Study Selection: Titles and abstracts of 1698 records were examined. After 1504 records were excluded, 2 authors independently assessed the full text of all remaining papers (n=194); disagreements were resolved by discussion. Data Extraction: A standardized form for assessment of study quality and evidence synthesis was used to extract data from the included studies. Results: Twenty studies were included in the systematic review, 14 of which were included in the meta-analysis. Meta-analyses showed that lower (vs higher) birth order and smaller (vs greater) number of siblings were associated with overweight and/or obesity, with ORs of 1.47 (95%CI, 1.12-1.93) and 1.46 (95%CI, 1.17-1.84), respectively. However, among the 9 studies that attempted to separate the effects of birth order and number of siblings in the same analysis, a higher risk of overweight/obesity was consistently found among individuals without siblings than among those with 1 or more siblings, rather than among firstborns more generally. Conclusion: The results show that both lower birth order and lower number of siblings are associated with risk of overweight/obesity, which suggests that only children are at a slightly increased risk of overweight/obesity. Systematic Review Registration: PROSPERO registration number CRD42014015135.