For the heavily glaciated mountains of southern Alaska, few high-resolution, millennial-scale proxy temperature reconstructions are available for comparison with modern temperatures or with the history of glacial fluctuations. Recent catastrophic drainage of glacier-dammed Iceberg Lake, on the northern margin of the Bagley Icefield, exposed subaerial outcrops of varved lacustrine sediments that span the period A.D. 442-1998. Here, an updated chronology of varve thickness measurements is used to quantitatively reconstruct melt season temperature anomalies. From 1958-1998, varve thickness has a weak positive correlation (r = 0.23) with May-June temperatures at the nearest coastal measurement stations. Varve sensitivity to temperature has changed over time, however, in response to lake level changes in 1957 and earlier. I compensate for this by log-transforming the varve thickness chronology, and also by calibrating the varve chronology against a 400 year long tree ring-based temperature reconstruction that spans the full range of occupied lake levels and varve sensitivities (r = 0.29). Reconstructed temperature anomalies have broad confidence intervals, but nominally span 0.5°C over the last 1500+ years. Maximum temperatures occurred in the late 20th century with a minimum in the late 6th century. The Little Ice Age is present as three cool periods between 1350 and 1850 with maximum cooling around 1650. A Medieval Warm Period is evident from A.D. 1000 to 1100, but the temperature reconstruction suggests it was less warm than recent decades – an observation supported by independent geological evidence of recent glacier retreat that is unprecedented over the period of record.