Manik Saha contested the elections for the first time in his three-decade political career.
Tripura Chief Minister Manik Saha, who won the recent by-election from Town Bardowali constituency, took oath as the member of the Tripura assembly on Friday.
Tripura assembly Speaker Ratan Chakraborty administered the oath of office and secrecy to Mr Saha.
The Chief Minister tweeted, "Took oath as a Member of Tripura Legislative Assembly, today. With the help of colleagues in the house and under the leadership of Hon'ble PM Shri Narendra Modi Ji, we will definitely take forward the development works of the state."
Mr Saha, who replaced Biplab Kumar Deb as the Chief Minister earlier in the year, was not a member of the Assembly.
According to Article 164(4) of the Constitution, an unelected member can assume office as CM but he or she has to win a byelection or get elected to the Legislative Council within six months to retain the chair.
Mr Saha resigned as Rajya Sabha member on Monday, paving the way for his swearing-in as MLA.
The 69-year-old Congress-turned-BJP leader contested the elections for the first time in his three-decade long political career and won from the Town Bordowali Assembly constituency.
In the June 23 by-polls, result of which was declared on June 26, the ruling Bharatiya Janata Party won three of the four seats including that of Mr Saha, who defeated Congress candidate Asish Kumar Saha by a margin of 6,104 votes.
The other three newly elected members -- Sudip Roy Barman (Congress) and Malina Debnath and Swapna Das (Paul), both BJP -- took oath as members of the Tripura assembly on June 28.