Na Kasem (Thai: นาเกษม) is a tambon (subdistrict) located in the easternmost area of Thung Si Udom district, in Ubon Ratchathani Province, Thailand.

[1] Neighbouring subdistricts are (clockwise from the south) Kut Ruea, Khok Chamrae, Som Sa-at, Non Sombun, and Kaeng.

[2] The area was settled by a group of villagers evacuated from Sisaket province; the first settlement was named "Kan Tuan" (กันต๊วน) but was later renamed "Na Kasem" after obtaining subdistrict status in 1979.

[3] The tambon was founded by incorporating seven settlements in the northern part of Kaeng in the Det Udom district.

[4] Later in 1992, tambon Na Kasem, together with four other subdistricts, including Thung Thoeng, Khok Chamrae, Nong Om, and Kut Ruea, were split off from Det Udom district to create a minor district (king amphoe) called Thung Si Udom.