"We have no doubt that you will carry out this task with responsibility and honor," President Patrice Talon said at ceremonies to mark the court's new term.
Dorothe Sossa, a former minister for justice and higher education who is a lawyer by training, will preside over the court.
Three members of the court have been appointed by President Talon, and the other four by the National Assembly, which is dominated by his supporters.
The Constitutional Court's responsibilities include supervising the next elections, due in 2026.
Talon, a cotton magnate who entered politics, has been in power since election in 2016 and was re-elected in 2021.
He has given priority to development, but opponents say the cost has come with creeping authoritarianism.
His tenure has coincided with a tightening of election rules that have dealt a blow to Benin's once thriving multi-party democracy.
In legislative elections in January, the opposition returned to the 109-seat single chamber after a four-year absence, picking up 28 seats while parties allied with Talon won 81.
The Democrats, the main opposition, reviled the choice of the judges named by the National Assembly's bureau. It said the party had had no chance of making any of the appointments.
In recent years, the Constitutional Court has issued a number of rulings touching on election organization and results, and opposition complaints have been mainly rejected.