Determining paleotemperatures in terrestrial environments are much more challenging than those in the ocean because of stratigraphic inconsistencies, strong spatial and temporal variations in temperature, and a paucity of well-tested methods. Here we utilize the ganoine scales of gars from the family Lepisosteidae to calibrate a new terrestrial paleothermometer. Gars are widespread both in the modern and in the past, as they are a freshwater fish lineage that extends back into the Cretaceous (100 Ma) and have remained relatively unchanged during that time span. Gars constantly record water temperatures, whose yearly average is closely related to mean annual temperature, in their body tissues, including scales. These scales grow continuously throughout life, are >95% hydroxyapatite and thus are highly resistant to diagenetic alteration. Oxygen isotopes in both biogenic phosphates and carbonates have been used to reconstruct environments on land with varying degrees of success. Phosphate-oxygen isotopes are more resistant to post-mortem alteration as the phosphorus-oxygen bond is stronger than the carbon-oxygen bond. We investigate the application of phosphate oxygen isotopes to gar scales by collecting scales from modern individuals from a north-south transect across the United States, exploiting the latitudinal temperature gradient in mean annual temperatures, measuring δ18Ophosphate of those scales, and comparing these values to the average δ18Owater and temperature of each locality. We compare our δ18Ophosphate calibration to previously published curves. Our work demonstrates that the δ18Ophosphate values of gar scales are robust recorders of temperature and δ18Owater.