The right-wing Bharatiya Janata Party and its local allies swept the polls, winning 43 seats in the 59 member house in remote Tripura, ending nearly 25 years of communist rule in the state that borders Bangladesh.
Elections in the three northeastern states -- Tripura, Nagaland and Meghalaya -- were held in February with results announced Saturday.
Modi, who campaigned vigorously in the region ahead of the vote, said the party charted "the path of success from (being) no one to the one" in the region, which for decades was considered a bastion of main opposition parties Congress and the Left Front.
"The northeast has today come forward to lead India on the path to development," Modi told a jubilant crowd of supporters at the victory speech in New Delhi party headquarters.
He termed the results as "epoch-making" and promised to "transform" the remote region.
The BJP itself bagged 35 out of 59 seats in Tripura, while its regional alliance partner Indigenous People's Front Of Tripura won eight seats.
That left the Communist Party of India (Marxist)-led Left Front with just 16.
Communists had won five consecutive elections in the state since 1993 -- three under the outgoing chief minister Manik Sarkar -- after wresting power from Congress.
Modi's Hindu nationalist party finished third in Christian-dominated Nagaland state, but was set to form the government in alliance with a local partner, party leaders said.
It won 11 seats on its own and was in talks with two other regional parties to form a coalition government.
BJP was a partner in the ruling alliance in Nagaland but had pulled out of the government ahead of the polls.
The BJP's succes has further consolidated the party's grip on national politics ahead of the crucial 2019 general elections, when Modi will be running for re-election.
Since winning the 2014 elections under Modi, the BJP has expanded its rule from seven states to 21, out of 29 Indian states.
It controls Manipur, Assam and Arunachal Pradesh states in the northeast region, which critically lacks infrastructure despite being of strategic importance due to its borders with China, Myanmar, Bhutan and Nepal.
However, the party could not garner support in Meghalaya, winning just two seats as the state heads for a hung mandate, with Congress winning the largest share with 21 seats in the 59 member house.
Congress has ruled the state since 2010, but it was unclear whether the Rahul Gandhi-led party will form a local alliance with other parties.
Elections are due in five more states later this year, with the BJP eyeing a victory in Congress-ruled southern Karnataka state that could reduce India's oldest political party's rule to just two states.
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