Meghalaya's newly-appointed Chief Minister Conrad Sangma and his counterparts from Arunachal Pradesh, Assam and Manipur were present at the swearing-in ceremony.
This will be Neiphiu Rio's fourth time as the Chief Minister of Nagaland. Mr Rio was the Chief Minister of Nagaland for three terms between 2003 and 2014, making him the only Nagaland Chief Minister to have served three consecutive terms.
Outgoing chief minister TR Zeliang resigned on Tuesday. Mr Zeliang's party, the Naga People's Front(NPF), had emerged the single largest party in the elections, winning 26 of the 60 assembly seats. The BJP-NDPP combine that bagged 30 seats has the support of one JD(U) legislator and an Independent.
The BJP had pulled out of an alliance with the NPF and chose to ally with Mr Rio's new party. Mr Rio said the alliance would be called "People's Democratic Alliance".
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.