The Governor said he had received Letters of Support in Mr Rio's favour from 12 legislators of the BJP, one letter from the JD (U) and another from an Independent MLA, besides the 18 legislatures of the NDPP.
The BJP leaders informed the Governor that Y Patton had been elected the BJP Legislature Party leader in the Assembly.
Along with 67-year-old Rio, 10 cabinet ministers were also administered the oath by governor PB Acharya.
Rio, who returned to state politics after a gap of nearly four years after resigning from the Lok Sabha in 2014, also said that his government would ensure that the "votes cast in his party's favour do not go in vain". Mr Rio informed that all the PDA alliance political parties have the same agenda in their manifestos for the solution of Naga political problem.
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.
The NDPP-BJP alliance, forged before the elections, managed to unseat the 15-year rule of the NPF as it garnered the support of the two MLAs to take their tally to 32 in the assembly.
Iterating one of the promises made by the NDPP in its election manifesto, Rio said his government will remove VIP culture from Nagaland and ensure that the citizens are treated as "real VIPs".
Outgoing Chief Minister TR Zeliang, who resigned on Tuesday, was also present.
The oath-taking ceremony was attended by Union Defence Minister Nirmala Sitharaman, Minister of State (Home) Kiren Rijiju, BJP President Amit Shah, BJP National General Secretary Ram Madhav, and Chief Ministers of Manipur & Assam.
The lone Janata Dal-United MLA, G Kaito Aye, and Independent legislator Tongpang Ozukum were also joined the Cabinet.
Rio is heading the Peoples Democratic Alliance government in partnership with the BJP in the state. In January 2018, Nagaland's lone Lok Sabha member broke away from the ruling NPF after the party chose to sever ties with the BJP.