The first chief minister in Nagaland to have won three elections in a row, Rio assumed office for the fourth time in the northeastern state, as saffron flags fluttered across the capital to underline his partys alliance with the BJP.
The former leader of the Naga People's Front (NPF), Rio, 68, who served as chief minister in 2003, 2008 and 2013, is one term short of the record set by S C Jamir, now Odisha governor, who headed a government five times.
His driver was killed and several of his bodyguards were injured.
Rio, who, according to the Association for Democratic Reforms is the richest legislator in Nagaland with assets worth over Rs 36 crore, belongs to the Angami Naga tribe.
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Born in 1952 to the Late Guolhoulie Rio of Tuophema village in Kohima district, his early education was at the Baptist English School (now Baptist High) in Kohima and then Sainik School in Purulia, West Bengal. He went for higher studies to St Joseph's College in Darjeeling and then to Kohima Arts College.
Rio contested as an independent candidate in the 1987 polls from Northern Angami-II Assembly seat, but was defeated.
He contested in 1989 on a Congress (I) ticket and was made Minister for Sports and School Education and subsequently became a Cabinet Minister for Higher and Technical Education, Art and Culture.
From the same constituency, he was re-elected as a Congress candidate in 1993 and then elected unopposed in the 1998 Assembly election. Earlier Minister for Works and Housing, he was appointed state Home Minister.
He joined the then almost-defunct Naga People's Council (NPC) and revived it. He re-christened it the Nagaland People's Front, now Naga People's Front (NPF).
Rio successfully contested from the Northern Angami -II seat in 2003 on an NPF ticket. He was elected leader of the Democratic Alliance of Nagaland (DAN) and appointed chief minister till President's Rule was imposed on January 3, 2008.
Re-elected from the same constituency as an NPF candidate in the 2008 Assembly election, he was again made leader of the NPF and DAN Legislature Party and appointed chief minister for the second term.
But he soon returned to state politics, and emerged as the unopposed winner from Northern Angami-II seat after NPF candidate Chupfuo Kense pulled out of the contest in the recent Assembly elections.
The National Democratic Progressive Party (NDPP), floated by Rio just before the Assembly elections, aligned with the BJP to form the People's Democratic Alliance government in the state.
While the NDPP won 18 seats, the BJP cornered 12 in the 60-member House.
The support of Independent candidate Tongpang Ozukum and JD(U)'s G Kaito Aye, who has been one of Rio's close aides, helped him oust incumbent chief minister T R Zeliang and pick up the reins of the state for the fourth time.
He was appointed chief minister for the third term on March 5, 2013, after getting elected from the same constituency as an NPF candidate.
Rio, who lists gardening and music as his hobbies, was also the chairman of the Northern Angami Area Council in 1984 and an honorary vice president of the Indian Red Cross Society in Nagaland.
The chief minister, who has travelled to 27 countries, is married to Kaisa Rio. They have five daughters and one son.