Whether childhood obesity or weight gain leads to the development of pediatric immune-mediated skin diseases remains unclear. We aimed to determine the associations between body mass index or body mass index changes and the development of 3 main immune-mediated skin diseases-alopecia areata, atopic dermatitis (AD), and psoriasis-by analyzing a longitudinal cohort of 2,161,900 Korean children from 2009 to 2020. The findings indicated that children who were obese had a higher risk of pediatric immune-mediated skin diseases than those with normal weight (P for trend < .01). An increase in body mass index was associated with a higher risk of AD, whereas a decrease in body mass index was correlated with a reduced risk of AD. Children who gained weight, transitioning from normal to overweight, exhibited a higher AD risk than those who maintained a normal weight (adjusted hazard ratio = 1.15, 95% confidence interval = 1.11-1.20). However, those who shifted from being overweight to achieving a normal weight (adjusted hazard ratio = 0.87, 95% confidence interval = 0.81-0.94) had a lower AD risk than children who were overweight who maintained their weight. In summary, early childhood obesity may increase the risk of pediatric immune-mediated skin diseases. Weight gain may increase AD risk, whereas weight loss may lower the risk.