Postcopulatory sexual selection, in the form of sperm competition, is a powerful selective force that is able to influence male's reproductive biology and sperm traits. This phenomenon occurs when females mate with different males in the same reproductive period and sperm from rival males compete in the female tract to fertilize the ova. Under these conditions, high swimming speed may be advantageous to negotiate barriers in the female reproductive tract and reach the site of fertilization. Thus, in a sperm competition scenario, high swimming speed may be regarded as an adaptation to maximize fertilization opportunities for a given male. Sperm cells show a considerable morphological variation across taxa. These differences are thought to be an important determinant of sperm swimming velocity although it is still unclear how changes in sperm morphology influence sperm velocity. This study used a new approach to understand the relationships between sperm head shape and swimming velocity assessing sperm morphology by means of geometric morphometrics. Our results show that different sperm head phenotypes, and overall sperm design, are clearly associated to differences in swimming velocity and, also, different swimming trajectories. In addition, results show that postcopulatory sexual selection exerts a clear influence shaping sperm head morphology. This implies that “fast sperm phenotypes” are favoured with increased levels of sperm competition. Thus, sperm head shape seems to be an important target of postcopulatory sexual selection and sperm modifications due to sperm competition may have an adaptive value enhancing fertilization success.