This study finds that immigrants who move to U.S. counties with climates very different from their country of birth tend to live shorter lives. Using linked census and Social Security death records, this study found that a larger temperature gap between an immigrant’s birthplace and their new home is linked to lower life expectancy, especially among people with less education and those in weather-sensitive jobs like farming and forestry. The study estimates that this “temperature mismatch” may have led to over 3 million lost life-years among immigrants living in the U.S. in 1940, raising important questions about how climate-driven migration could shape public health in the decades ahead.