Padu, who had contested from Aalo West constituency and was defeated by 30 votes, had challenged the result in the Court accusing the winner of resorting to unfair means and manipulating the counting of votes.
She also alleged that 65 postal ballots mysteriously disappeared from Aalo post office and resurfaced at the time of counting of votes.
With this judgement passed on Friday, the strength of Trinamool Congress rose to six in the 60-member assembly.
Padu will be the first woman MLA from Galo community, one of the largest tribes of Arunachal Pradesh.
Earlier, the Supreme Court had dismissed two appeals made by Ete pleading the apex court to dismiss two election petitions filed by Padu challenging his election.