Climate change is causing not only steady increases in average global temperatures but also an increasing frequency with which extreme heating events occur. These extreme events may be pivotal in determining the ability of organisms to persist in their current habitats. Thus, it is important to understand how the rate of gain and loss of heat tolerance by organisms compares with the frequency of extreme heating events in the field. We show that the California mussel, Mytilus californianus —a sessile intertidal species that experiences extreme temperature fluctuations and cannot behaviorally thermoregulate—can quickly (in 24-48 h) heat acclimate after exposure to a single sublethal heat-stress bout (2 h at 30 or 35°C) and can maintain this improved tolerance for up to ~3 weeks without further exposure to elevated temperatures. This adaptive response improved survival rates under extreme heat stress bouts (2 h at 40°C) by ~75%. To apply these lab findings in an ecological context, we evaluated 4 y of mussel body temperatures recorded in the field. The majority (~64%) of heat stress bouts are separated by 24-48 h, but were at times separated by as much as 22 days. Thus, the ability of M. californianus to maintain improved heat tolerance for up to 21 days after a single sublethal heat-stress bout significantly improves their probability of survival, as ~36% of consecutive heat events are separated by 3-22 days. We show that mussels have evolved a highly adaptive strategy to survive their habitat, whereby quickly gaining, and slowly losing, heat tolerance promotes survival. This strategy will likely allow mussels to survive the intermittent and extreme heat events predicted with climate change.