Tvarditsa Municipality (Bulgarian: Община Твърдица) is a municipality in the Sliven Province of Bulgaria.

Ethnic composition of Sliven Municipality At the 2011 census, the population of Tvarditsa was 13,804.

Most of the inhabitants were Bulgarians (67.79%) with a minority of Turks (9.32%) and Gypsies/Romani (15.21%).

[1][2] 7.07% of the population's ethnicity was unknown.

In addition to the capital town of Tvarditsa, there are 9 villages in the municipality: