The ancient lake-bed of the Florissant Formation in Colorado, which closely precedes the Eocene-Oligocene climatic transition (~34.1 Ma), preserves a diverse assemblage of flora and fauna. There are many published estimates of mean annual temperature (MAT) for Florissant, including over 15 based on the taxonomy and leaf physiognomy of plants, which range from ~10-18 °C. Here we use digital leaf physiognomy (DiLP) to reconstruct MAT. DiLP is based on traits related to leaf dissection, including the size and frequency of teeth. Similar to other leaf physiognomy-based climate proxies, DiLP tacitly assumes a convergent morphological adaptation within woody dicotyledons to a shared climate space. Our analysis is based on 174 fossil leaves from 64 species and five paleobotany collections across the United States, highlighting the importance of historic collections and their critical role in ongoing research. We estimate a MAT for Florissant of 12.4 ± 4 °C, which falls between the lower temperatures predicted by Climate-Leaf Analysis Multivariate Program (CLAMP) and the generally higher estimates based on taxonomy (nearest living relatives). Our estimate provides context for understanding the Florissant ecosystem and more generally for the Colorado region just before the onset of the Eocene-Oligocene transition.