Clay County Population 2018
Clay County's estimated population is 13,990 with a growth rate of 0.37% in the past year according to the most recent United States census data. Clay County is the 15th largest county in South Dakota.
| Year | Population | Growth | Growth Rate |
| 2017 | 13,990 | 52 | 0.37% |
| 2016 | 13,938 | 148 | 1.07% |
| 2015 | 13,790 | -111 | -0.80% |
| 2014 | 13,901 | -13 | -0.09% |
| 2013 | 13,914 | -133 | -0.95% |
| 2012 | 14,047 | 60 | 0.43% |
| 2011 | 13,987 | 157 | 1.14% |
| 2010 | 13,830 | 644 | 4.88% |
| 1990 | 13,186 | -503 | -3.67% |
| 1980 | 13,689 | 766 | 5.93% |
| 1970 | 12,923 | 2,113 | 19.55% |
| 1960 | 10,810 | -183 | -1.66% |
| 1950 | 10,993 | 1,401 | 14.61% |
| 1940 | 9,592 | -496 | -4.92% |
| 1930 | 10,088 | 434 | 4.50% |
| 1920 | 9,654 | 943 | 10.83% |
| 1910 | 8,711 | -605 | -6.49% |
| 1900 | 9,316 | 1,807 | 24.06% |
| 1890 | 7,509 | 2,508 | 50.15% |
| 1880 | 5,001 | 2,380 | 90.81% |
| 1870 | 2,621 | 0.00% |