This study aimed to determine the effect of the presence of under-5 siblings (⩾ 1) in a household on childhood malnutrition in urban Bangladesh. During 2000 and 2013, a total of 16,948 under-5 children were enrolled in the Diarrhoeal Disease Surveillance of icddr,b. Under-5 siblings were categorised as ⩾ 1 and none except the child himself. In univariate analysis, the presence of siblings was associated with 1.13 (risk ratios=1.13; 95% CI:1.06-1.20) times higher risk of being stunted, 1.17 (1.09-1.25) times for wasted and 1.19 (1.13-1.26) times underweight compared with their peers who did not have siblings. In multivariate analysis, such associations remained significant for stunting (1.08; 1.01-1.15), wasting (1.12; 1.04-1.21) and underweight (1.13, 1.06-1.19) after controlling for possible confounders such as age of child, sex, parental education, maternal employment, family size, wealth quintile and time (year). The presence of under-5 siblings increases the risk of malnutrition in children in urban Bangladesh.