Ocean acidification may have deleterious effects on many species, but anticipating long-term changes in the abundance of populations will require an understanding of ocean acidification as an evolutionary force. Here I show that ocean acidification alters natural selection on offspring size and is likely to drive contemporary evolution. In a detailed study of a coastal fish species (California Grunion), I demonstrate that larval mortality is highly sensitive to ocean acidification and that mortality rates are lower for larger larvae. However, these effects are countered by tradeoffs between offspring size and number, suggesting that measurements of maternal fitness are critical for quantifying selection through ocean acidification. Measurements of selection and genetic variation were used to project the evolution of larval size as seawater conditions changed incrementally over many decades. Results for California Grunion suggest that contemporary evolution may offset the projected decline in reproductive success by about 50%.