Elwin is an unincorporated community in the city of Decatur, Illinois, located in Macon County, Illinois. The community is located on U.S. Route 51 in the central portion of the county. Elwin was named after its founders Elwood and Martin. As of 2009, Elwin's population is 163 people. Since the 2000 United States Census, it has had a population growth of -1.72 percent.