As of 2001[update] India census, Samthar had a population of 20,227. Males constitute 53% of the population and females 47%. Samthar has an average literacy rate of 55%, lower than the national average of 59.5%: male literacy is 66%, and female literacy is 43%. In Samthar, 16% of the population is under 6 years of age.
Samthar was formerly known as Samshergarh. Samthar State was an independent principality governed by Gurjars of the Khatana clan who had migrated from Rajasthan. The independent state of Samthar was created by Chandrabhan Vir Gujar and his grandson Madan Singh, governors of the state of Datia.