In a statement, ECOWAS said the suspension followed a decision adopted at its 64th session, held in the Nigerian capital of Abuja on December 10.
Mohammed Bazoum, who was overthrown by the country's presidential guard in late July, remained the democratically elected president, according to the statement.
The military in Niger detained Bazoum on July 26 and chose Abdourahamane Tchiani, former leader of the country's presidential guard, to lead the CNSP, a governing body established by the soldiers after the coup, which has been since exercising legislative and executive authority.