The Interior Ministry says the woman, who was planning to target tourist areas in the country, is under arrest.
Tunisian police have thwarted an attack planned by a woman coming from Syria, where she received training "with terrorist groups".
The country's Interior Ministry said on Friday the woman, who was planning attacks on tourist areas with an explosive belt, had been imprisoned.
The ministry said the woman returned to Tunisia on January 10 after a year of training in Syria, where she planned the attack.
Tunisian security forces have thwarted most militant plots in recent years and they have become more efficient at responding to those attacks that do occur, Western diplomats say.
In November, police shot and wounded an extremist who sought to attack them with a knife and cleaver in the capital.
The last major attacks in Tunisia took place in 2015 when militants killed scores of people in two separate assaults at a museum in Tunis and a beach resort in Sousse.