Phelps Mill is a flour mill in Otter Tail County, Minnesota, United States, on the Otter Tail River. The mill was built in 1888–1889 by William E. Thomas, a local entrepreneur who owned a flour and feed business in Fergus Falls. During that time, wheat was a high-demand crop, and nearly one thousand flour mills were in operation throughout Minnesota. Thomas began constructing a wooden dam on the river in the spring of 1888, although the dam was prone to leakage and had to be shored up with sandbags, dirt, gravel, and other materials. The mill itself was built by Royal Powers, who built and framed the mill without using blueprints. He was able to keep the entire plan within his head and did not even have to mark out the lumber he was cutting.