The BJP secured more than 250 seats in the 403-member state legislature — by far the largest in the country. It was the first time in more than three decades that a party has won a second consecutive term in the state.
UP, as the state is often called, was the biggest prize among the five regions where voting was held over the past month.
Home to a population of 200 million people, UP is seen as a bellwether in Indian politics. Winning the state could boost the chances of Modi and his party scoring a third straight term office in the 2024 general election.
Niranjan Sahoo, senior fellow at New Delhi-based think tank Observer Research Foundation, told DW that if the BJP sweep all five states, it would "remain in the driver's seat for 2024 national polls."
"It once again elevates PM Modi to enviable position of unchallenged leader of the saffron party. But its a bad news for the main opposition Congress, which is struggling to hold fort in its traditional bastion," Sahoo added.
All eyes in India on Uttar Pradesh election
Newbie party wins Punjab
Four smaller states were also counting votes cast during the month-long election season.
The BJP, as predicted by exit polls, performed poorly in the state, failing to get more than a handful of seats.
The Aam Aadmi Party (AAP), a relatively new party that currently governs the national capital Delhi, is set for a landslide in Punjab. It is leading in more than 90 out of the total 117 seats.
"AAP's unprecedented sweep in a full fledged northern state makes them a national entity from a city-based party. From a political start-up in less than 8 years which began in Delhi city, it has come a long way," Sahoo said.
Punjab was previously led by the Congress party, India's main opposition at the national level, which also failed to make significant gains elsewhere.
BJP is also projected to win in the Himalayan state of Uttarakhand and is leading as the single-largest party in the tourist hotspot Goa, and the northeast state of Manipur.