Clerk to the Senate, Nelson Ayewoh, conducted the swearing-in ceremony after which Okorocha signed necessary documents.
Mr. Okorocha, representing Imo West, took the oath of office on Thursday morning.
The former governor’s certificate was initially withdrawn after the electoral umpire, INEC, said the issuing officer declared the result under duress after the February 23 senatorial elections.
On the day of inauguration of the ninth Senate, Okorocha made himself available in the morning but was not sworn in.
INEC’s insistence on not issuing the certificate was despite a court order directing the commission to do so.
However, on Tuesday, INEC decided to issue Okorocha his certificate, but said it would appeal the court ruling.