The PDA includes 18 members from NDPP, 12 from BJP, one from the Janata Dal-United and an Independent in the 60-member Nagaland Assembly.
Rio previously held the office in 2003-08, 2008-13 and 2013-14, making him the only Chief Minister of Nagaland to have served three consecutive terms.
He was elected from the state's lone Lok Sabha seat in 2014.
Rio resigned as MP after he was elected unopposed as a member of the Assembly last month from the Northern Angami-II seat in Kohima district.
Rio will prove his majority on the floor of the House on or before March 16.
The new Chief Minister told IANS that the PDA will ensure good governance and draw a new roadmap for Nagaland's development under Prime Minister Narendra Modi's growth agenda.
Present at the oath taking ceremony were Defence Minister Nirmala Sitharaman, Minister of State for Home Kiren Rijiju, BJP President Amit Shah, BJP General Secretary Ram Madhav along with the Chief Ministers of Assam (Sarbananda Sonowal), Arunachal Pradesh (Pema Khandu), Manipur (N. Biren Singh) and Meghalaya (Conrad Sangma).
Outgoing Chief Minister T.R. Zeliang, who resigned on Tuesday, was also present.
Senior BJP legislators Paiwang Konyak, Jacob Zhimomi, S. Pangnyu Phom, V. Kashiho Sangtam and Temjem Imna Along were sworn in as Cabinet ministers.
The lone Janata Dal-United MLA, G. Kaito Aye, and Independent legislator Tongpang Ozukum were also joined the Cabinet.
The NDPP-BJP, which stitched a pre-poll alliance and contested in 40 and 20 Assembly seats respectively, formed the PDA with the Janata Dal-United, National People's Party and Independent member after the election results were declared on March 3.
The tribal-centric NPP, which won two seats in Nagaland, withdrew its support to the PDA coalition saying they were yet to obtain endorsement from its party Pesident Conrad Sangma.
The NPF won 27 seats in the 60-member Nagaland Assembly.
On Monday, the BJP announced it had severed its 15-year-old alliance with the NPF.
Business Standard has always strived hard to provide up-to-date information and commentary on developments that are of interest to you and have wider political and economic implications for the country and the world. Your encouragement and constant feedback on how to improve our offering have only made our resolve and commitment to these ideals stronger. Even during these difficult times arising out of Covid-19, we continue to remain committed to keeping you informed and updated with credible news, authoritative views and incisive commentary on topical issues of relevance.
We, however, have a request.
As we battle the economic impact of the pandemic, we need your support even more, so that we can continue to offer you more quality content. Our subscription model has seen an encouraging response from many of you, who have subscribed to our online content. More subscription to our online content can only help us achieve the goals of offering you even better and more relevant content. We believe in free, fair and credible journalism. Your support through more subscriptions can help us practise the journalism to which we are committed.
Support quality journalism and subscribe to Business Standard.