Adeyemi is replacing Senator Dino Melaye of the Peoples Democratic Party (PDP) who lost the rerun in the Kogi West senatorial district election.
The swearing-in was done by the Clerk of the Senate, Nelson Ayewoh, at the start of plenary.
Adeyemi is returning to the Senate about five years after he lost the seat to Melaye at the 2015 election.
Meanwhile, Dino Melaye vowed to challenge the result of the election at the election petition tribunal, insisting that he defeated Smart Adeyemi.
The former senator also claimed that there were five assassination attempts on his life and that government institutions were used against him.