Projected warming of global surface air temperatures will further exacerbate droughts, wildfires, and other agents of ecosystem stress. We use latewood blue intensity from high‐elevation Picea engelmannii to reconstruct late‐summer maximum air temperature for the Greater Yellowstone Ecoregion (GYE) spanning 770-2019 CE. Using a robust regression model (r2 = 0.60), the 1,250‐year reconstruction reveals 2016 as the single‐warmest year and the warming trend since ca. 2000 as the most intense. The Medieval Climate Anomaly contained the highest‐ranking warm event (1050-1070 CE) and was characterized by substantial multidecadal variability rather than a period of prolonged, homogeneous warming. We document regional expression of past warm and cool events, such as an anomalously warm period spanning the fifteenth to sixteenth centuries, and the Maunder and Dalton minima of the Little Ice Age. Summer temperature variability across the GYE shows multicentennial agreement with trends in solar irradiance, volcanic activity, snowpack, and other regional‐to‐hemispheric temperature records.