As of the census of India 2001, Kuzhithurai had a population of 20,503 comprising 10,069 males and 10,434 females, making the sex ratio (number of females per thousand males) of the town to 1,036. A total of 2,133 people were under six years of age and the child sex ratio (number of females per thousand males under six years of age) stood at 968. The town had an average literacy of 90.97%, higher than the national average of 59.5%. A total of 341 comprising 1.86% of the population belonged to Scheduled Castes (SC) and 27 comprising 0.15% of the population belonged to Scheduled tribes (ST). There were are total of 4,437 households in the town. As of 2001, Kuzhithurai had a total of 5,258 main workers: 57 cultivators, 241 agricultural labourers, 178 in house hold industries and 4,782 other workers. There was a total of 1,033 marginal workers: 8 marginal cultivators, 111 marginal agricultural labourers, 117 marginal workers in household industries and 797 other marginal workers.
People of this town generally know both Tamil and Malayalam languages, because of its proximity to the border state of Kerala. Tamil and Malayalam are widely spoken by the people. The culture is a mixture of Tamil and Malayalam culture and traditions. Hinduism and Christianity are the major religions in the town.