Reconstructions of coldest month mean temperature (CMMT) in the geologic past are important for understanding the dynamics of the climate system operating at that time. A common method for estimating CMMT involves applying the climatic tolerances of modern plant species to close analogues in the fossil record (the so-called nearest living relative [NLR] approach). Here we show that the leaves of seven modern species commonly used as NLRs to reconstruct CMMT from the fossil record freeze at warmer temperatures when grown at high CO2 (800 vs. 400 ppmv). This CO2 effect was found across all tested plant groups, including palms, taxodiaceous conifers, and two Southern Hemisphere evergreen species. This pervasive response indicates that many NLR-derived estimates of CMMT for times of high CO2 should be adjusted by at least +1.5 to +3 °C. A revision of CMMT estimates toward warmer values further increases the disparity in continental interiors between CMMT indicators from the fossil record and the colder predictions from general circulation models.