The results paved the way for the outspoken leader to take the state chief minister's office for the fourth time.
Mayawati's legislators started arriving in Lucknow, the state capital, for a meeting at which she was due to be formally elected to lead her party in the provincial house.
The victory surprised her rivals and contradicted exit polls.
"People of all castes have brought us to power," Mayawati told a press conference, as she thanked the upper-castes and Muslims for spurning their traditional preferences to vote for her.
"The people of the state have also proved they believe in democracy and I promise we will unshackle the millions from a regime of fear, injustice and will offer them full democratic rights," she said.
News of Mayawati's win made front-page headlines in all the leading dailies, which noted that Uttar Pradesh would have a majority government for the first time in 14 years, after being run by a string of often unstable coalitions.
"It's nothing but Maya," read the headline in The Times of India. Maya is Hindi for illusion.
"Mayawati's mandate in Uttar Pradesh is spectacular. It confirms that despite its inadequacies, India's democracy remains exceptional," said the Indian Express in an editorial, commending her on the success.