The 71-year-old Sai succeeds Rameshwar Oraon, whose tenure had ended on October 31 last year.
Taking charge, Sai said he would work to protect the rights of the tribal people living in far-flung areas of the country.
"A large number of tribals in our country are still not aware of the rights guaranteed to them in the Constitution. We will see to it that NCST is converted into an important tool for the overall social economic development of STs in the country," he said.
Sai was elected to the Madhya Pradesh legislative assembly in 1977, 1985 and 1998. He was also elected to the Chhattisgarh legislative assembly in 2000 and was the first leader of opposition in the House.
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Sai was elected to the Lok Sabha in 1989, 1996 and 2004 and was also elected to the Rajya Sabha in 2009 and 2010. He has also served as member of parliamentary standing committee on coal and steel and member of the parliamentary consultative committee of Ministry of Urban Development and Ministry of Social Justice and Empowerment.
The NCST is empowered to investigate and monitor matters relating to safeguards provided for STs under the Constitution.
The Commission is also authorised to inquire into specific complaints relating to the rights and safeguards of STs and to participate and advise in the planning process relating to their socio-economic development.
The Commission submits its report to the President annually on the working of safeguards and measures required for effective implementation of programmes and schemes relating to welfare and socio-economic development of STs.