South Plainfield, NJ has a population of 23,963 and is the 1,821st largest city in the United States. The population density is 2,888 per sq mi which is 288745% higher than the New Jersey average and 3088% higher than the national average. The median age in South Plainfield is 41 which is approximately 4% higher than the New Jersey average of 40. In South Plainfield, 62% of the population over 15 years of age are married, 66% speak English and 14% speak Spanish. 58% of South Plainfield residents were born in New Jersey, 15% were born out of state, 2% were born outside of the United States and 25% were foreign born.