Fossils from Paleogene High Arctic deposits provide some of the clearest evidence for greenhouse climates in the past and offer the potential to improve our understanding of Earth system dynamics in a largely ice-free world. One of the most well-known and stunningly preserved polar forest sites, Napartulik, crops out of middle Eocene (47.9–37.8 Ma) sediments on eastern Axel Heiberg Island, Nunavut, Canada (∼78°N paleolatitude). An abundance of data from Napartulik suggests mean annual temperatures at least 30 °C warmer than today and atmospheric water loads 2× current levels. Despite this wealth of paleontological and paleoclimatological data, there are currently no direct constraints on atmospheric CO2 levels for Napartulik or any other polar forest site. Here we apply a new plant gas-exchange model to Metasequoia (dawn redwood) leaves to reconstruct atmospheric CO2 from six fossil forest horizons at Napartulik. Individual reconstructions vary between 392 ppm and 474 ppm, with a site median of 424 ppm (351–523 ppm at 95% confidence). These estimates represent the first direct constraints on CO2 for a High Arctic forest and suggest that the temperate conditions present at Napartulik during the middle Eocene were maintained under CO2 concentrations ∼1.5× pre-industrial levels. Our results support the case that long-term climate sensitivity to CO2 in the past was sometimes high, even during largely ice-free periods, highlighting the need to better understand the climate forcings and feedbacks responsible for this amplification.