Saha had been elected to the state assembly after the by-polls held on June 23 in Tripura.
The speaker of the Tripura assembly, Ratan Chajraborty administered the oath of office and secrecy to the Chief Minister.
Meanwhile, CM Saha resigned from the Rajya Sabha on Monday ahead of his swearing in.
Taking to Twitter, the Tripura CM wrote, “Took oath as a Member of Tripura Legislative Assembly, today. With the help of colleagues in the house and under the leadership of PM Narendra Modi, we will definitely take forward the development works of the state.”
Having contested the elections for the first time in his three-decade-long political career, the former Congress leader Saha won from the Town Bordowali assembly constituency.
Saha, who is 69 years old, is also the chief of the state Bharatiya Janata Party (BJP) unit. He had taken oath as the Chief Minister on May 15, a day after the resignation of Biplab Kumar Deb.
He had to become a member of the assembly within six months to fulfill the constitutional obligation.
The results of the by-polls were declared on June 26 where the ruling BJP won three of the four seats. Manik Saha defeated Congress candidate Asish Kumar Saha by 6,104 votes.
It may be noted that the other newly elected members, Sudip Roy Barman of Congress, Malina Debnath and Swapna Das Paul of BJP, had taken oath as members of the Tripura assembly on June 28.