Niger is officially taking ownership of the disputed island of Lete located on the banks of the Niger River, where its flag is to be raised shortly.
Niger and Benin have disputed the island for decades but the disagreement was finally settled in the International Court of Justice in 2005.
Lete is some 60km square and its floods plains are used by nomads for grazing.
There are also unconfirmed reports that the island may be rich in oil and other minerals like iron.
The Niger River forms a natural border between Niger and Benin.
The BBC's Idy Baraou, who is attending the ceremony, says hundreds of soldiers, republican guards, gendarmes, and university scholars and lecturers are expected at the event.
Three government ministers will conduct the ceremony, he says.
Our correspondent says the case was referred to the ICJ in 2000 after the two countries closed their borders following a raid by Niger's army, in which infrastructure built by Benin was destroyed.
The ICJ based its rulings on grazing permits awarded by the French colonial authorities in 1914.
Both countries became independent in 1960 but they did not agree on sovereignty over Lete and 15 smaller islands.
The ICJ awarded Benin ownership of nine of the other islands.