Rising atmospheric carbon dioxide (CO2) concentrations are driving rapid changes to seawater chemistry, resulting in decreased ocean pH and carbonate ion concentrations. The rate and magnitude of change is unprecedented in the geological record, hence ocean acidification (OA) will place significant novel selection on marine taxa. Sperm shed directly into the water column for external fertilization may be particularly vulnerable to OA due to their limited buffering capacity to seawater pH change. However, the reproductive consequences of OA are poorly understood, particularly in the context of sperm competition, where ejaculates from multiple males compete to fertilize a batch of eggs. Sperm competition is extremely common, hence this knowledge gap significantly limits our understanding of the consequences of OA for many large and ecologically important marine taxa. Here, we investigated the impacts of simulated OA conditions (+1000 µatm pCO2) on sperm competitiveness for the sea urchin Paracentrotus lividus. Males with faster sperm had greater competitive fertilization success and the relationship between sperm speed and reproductive success held across seawater conditions. Similarly, males with more motile sperm had higher fitness, but only under current seawater pCO2 levels. Under OA the strength of this association was significantly reduced and there were also male sperm performance rank changes under OA, such that the best males now are not necessarily best under OA. These changes could alter the identity and diversity of males securing paternity in future acidified oceans and the consequences of this are far from clear.