Anthropogenic carbon dioxide (CO2) emissions and the resultant acidification of surface ocean waters are predicted to have far-reaching consequences for biological processes in the marine environment. For example, because changes in pH and pCO2 can alter sperm performance, ocean acidification may be accompanied by reductions in the success of fertilization in marine broadcast spawners. Several studies have attempted to determine the effects of elevated pCO2 on marine invertebrate fertilization success, albeit with differing results. These conflicts may stem from the use of inappropriate sperm–egg contact times and, in several cases, the lack of measurements over a range of sperm concentrations extending from sperm-limited conditions to polyspermy scenarios. In our study, we used biologically realistic sperm–egg contact times and a full range of sperm concentrations to assess the effect of elevated pCO2 on fertilization in the broadcast spawning sea urchin, Strongylocentrotus franciscanus. Fertilization experiments were carried out in seawater bubbled with CO2 to 400 (control), 800, and 1800 ppm. Using a fertilization kinetics model, we estimate that elevated pCO2 levels both increased sperm limitation and reduced the efficiency of fast blocks to polyspermy. Thus, elevated pCO2 decreased the range of sperm concentrations over which high fertilization success was likely. Given the inherent difficulties in achieving high fertilization success in broadcast spawners, raised pCO2 levels are likely to exacerbate low fertilization success in low-density populations or in areas with high water turbulence.