Installed in 29-04-1878. Raised to the condition of city and headquarters of the city with the denomination of Alagoa de Baixo, for the state law n 991, of 01-07-1909. For the municipal law of 15-10-1909, the annexed district of Safekeeping and to the city of Alagoa de Baixo is created. In referring administrative division to the year of 1911, the city is constituted of 2 districts: Alagoa de Baixo and Custdia.
For the state law n 1931, of 11-09-1928, desmembra of the city of Alagoa de Baixo the district of Safekeeping. Raised to the category of city. By the municipal laws ns 39, of 02-04-1919 and 96, 18-01-1929, is created the district of Cottons and annexed to the city of Alagoa de Baixo. In referring division to the year of 1933, the city is constituted of 2 districts: Alagoa de Baixo and Algodes. In divisions of 1936 and 31-XII-1937, the city Appears constituted of 4 districts: Alagoa de Baixo, Cottons, Enrique Days former-Tiger and Rio of the Bar. For the state Decree n 952, of 31-12-1943, the city of Alagoa de Baixo started to call Sertnia. In dated territorial division of 1-VII-1950, the city Already called Sertnia the city is constituted of 4 districts: Sertnia, Cottons, Enrique Days and River of the Barra.Por the municipal law n 133, of 14-02-1953, is created the district of Albuquerque N, created with lands of the district of River of the annexed Bar and to the city of Sertnia. In dated territorial division of 1-VII-1960, the city is constituted of 5 districts: Sertnia, Albuquerque N, Cottons, Enrique Days and River of the Bar, remaining in dated territorial division of 2005, with municipal toponmica alteration of Alagoa de Baixo for Sertnia, for the state Decree n 952, of 31-12-1943. The people who are born in Sertnia are sertanienses.