Anthropogenic climate change has already altered the timing of major life history transitions, such as the initiation of reproduction. Both phenotypic plasticity and adaptive evolution can underlie rapid phenological shifts in response to climate change but their relative contributions are poorly understood. Here, we combine a continuous 38-year field survey with quantitative genetic field experiments to assess adaptation in the context of climate change. We focused on Boechera stricta (Brassicaeae), a mustard native to the U.S. Rocky Mountains. Flowering phenology advanced significantly from 1973-2011, and was strongly associated with warmer temperatures and earlier snowmelt dates. Strong directional selection favored earlier flowering in contemporary environments (2010-2011). Climate change could drive this directional selection, and promote even earlier flowering as temperatures continue to increase. Our quantitative genetic analyses predict a response to selection of 0.2 to 0.5 days acceleration in flowering per generation, which could account for more than 20% of the phenological change observed in the long-term dataset. However, the strength of directional selection and the predicted evolutionary response are likely much greater now than even 30 years ago because of rapidly changing climatic conditions. We predict that adaptation will likely be necessary for long-term in situ persistence in the context of climate change.