The Lagos lawmaker defeated his opponent, Mohammed Bago, with over 200 votes in the house were 358 members voted.
The Independent National Electoral Commission (INEC) has reportedly issued Rochas Okorocha, Imo West senator-elect, his certificate of return.
He defeated Ike Ekweremadu of the PDP to clinch the position.
Mr Ekweremadu who belongs to the Peoples Democratic Party (PDP) told his colleagues not to elect a mace snatcher for the position.
Mr Lawan defeated Ali Ndume in the election conducted by the management of the National Assembly to emerge head of the red chamber.
Mr Lawan who was elected by his colleagues on Tuesday at the red chamber defeated Ali Ndume, another lawmaker from his party, to emerged the leader of the National Assembly.
The secret ballot system was used in 2015 to elect the immediate past leaders of the 8th National Assembly, the system is said to be used in today's election.
A photograph showing the senator-elect with a certificate has been trending online yesterday but the former governor has described it as fake.
Mr Kalu in a statement said his decision to step down is in the interest of the ruling party and the country at large.
The former governor also said his stay at the red chamber was that of sadness and happiness.