Paleohydrologic records provide a valuable perspective on the variability of streamflow and hydroclimate that is critical for water resource planning and placing present day and future conditions into a long-term context. Until now, key insights gained from streamflow reconstructions in the other river basins across the Western U.S. have been lacking in the Milk and St. Mary River Basin. Here we utilize a new database of naturalized streamflow records for the Milk and St. Mary Rivers and an expanded network of tree-ring records from the region to reconstruct streamflow at eight gaging locations located in the mountains, foothills, and plains reaches of the basins. The network of streamflow reconstructions presented here were generated for use by the Bureau of Reclamation and Montana Department of Natural Resources and Conservation in the Basin Study update for the Milk and St. Mary Rivers, and provides important data resources to water managers balancing increasing water demands for hydropower, irrigation, and ecological resources with increasing drought and flood risk in the basin.