Diatom communities preserved in sediments are indicators of historical temperature change. As part of a cross-regional comparison of post-industrialization climate change, we cored the sediments of a glacial lake in Alaska. Our working hypothesis for this lake was the dominant indicator diatoms would only show a response to climate change over the two most recent centuries. However, our analyses suggest the lake has gone through a series of temperature changes over the last two millennia. Between 400-600 AD, the diatom community became increasingly dominated by aerophilic and endosymbiotic species suggesting a decrease in lake temperature. Conversely, between 600-900 AD, the diatom community changed and included higher proportions of planktonic species and certain species of Fragilaria, suggesting an increase in temperature with an apex at 900 AD. Following a brief cooling period, lake temperatures warmed again between 1000-1300 AD, coinciding with the Medieval Climate Optimum. Beginning in 1300 AD, temperatures cooled until 1850 AD, with a cold temperature nadir during 1650 AD, coinciding with the Medieval Climate Minimum. After 1850 AD, the diatom communities shifted, suggesting another period of warming that may continue into the future. Our results are corroborated by another study in the region that used multiple stable isotopes.