Watertown, WI has a population of 23,878 and is the 1,831st largest city in the United States. The population density is 1,984 per sq mi which is 2169% higher than the Wisconsin average and 2090% higher than the national average. The median age in Watertown is 38 which is approximately 4% lower than the Wisconsin average of 39. In Watertown, 57% of the population over 15 years of age are married, 94% speak English and 4% speak Spanish. 79% of Watertown residents were born in Wisconsin, 18% were born out of state, 0% were born outside of the United States and 2% were foreign born.