We used molecular sexing and morphological analysis to characterise sexual size dimorphism (SSD) in the European Storm Petrel Hydrobates pelagicus (Linnaeus, 1758) in the southernmost and westernmost breeding colony (El Hierro, Canary Islands).We analysed SSD in 79 individuals (46 males, 33 females). We performed discriminant analysis on 9 morphometric variables and correctly classified 70.5% of the original cases. By binary logistic regression, the accuracy in sexing birds previously sexed by DNA analysis was 70.5%. Females were larger than males in wing and tarsus length but there were no differences for bill size and shape. There was a large biometric overlap between sexes in this Canarian breeding colony of H. pelagicus. However, a moderate female-biased dimorphism in flight-relevant variables such as tail and wing traits could be of interest in trophic segregation of sexes and optimization of resource use in these pelagic-feeding birds.