The 121-seat body known as the National Transition Council on Saturday elected Diaw after meeting for its inaugural session in the capital Bamako.
Colonel Malick Diaw was elected president unopposed with with 111 votes in his favour and seven abstentions.
He was second in command of the military junta that took power after Keita’s ouster. The junta has never formally been dissolved.
On Friday, the opposition June 5 Movement, which led protests against Keita this year, said it was boycotting the new legislature. It said it would not serve as a “stooge” for a disguised military regime”.
According to a government decree, members of the defence and security forces have 22 seats in the transition council, while political parties, civil society groups and trade unions also have seats.