According to the Mosaique FM radio station, the arrest occurred in the northwestern city of Jendouba, following a police raid on the house of the two brothers, where a computer with materials on how to make explosives and extremist propaganda was found.
The two suspects confessed that they had converted to Islam recently, the broadcaster said.
Tunisia has suffered sporadically from militant violence since the 2011 revolution, when long-standing president Zine El Abidine Ben Ali was ousted.
In 2015, Tunisia witnessed a number of notorious extremists attacks. In particular, in March 2015, a group of gunmen dressed in military uniforms opened fire on tourists at the Tunisian National Bardo Museum near the parliament building, killing 22. In June, 2015, 38 people were killed and 40 more wounded in the resort town of Sousse after a gunman disguised as a tourist opened fire on a beach. The Daesh jihadist group outlawed in Russia, claimed responsibility for both attacks.
A nationwide state of emergency was imposed following a deadly bomb attack in November 2015, when a bus carrying Tunisian presidential guards exploded, killing at least 12.