The merger of First Niagara Bank into KeyBank has received the go-ahead from the Office of the Comptroller of the Currency (OCC).
The cash-stock deal worth about $4.1bn was announced in October 2015. First Niagara will merge into KeyCorp as part of the transaction.
KeyCorp CEO and chair Beth Mooney said: "As we bring KeyBank and First Niagara together, at every turn we have worked to make this transition as smooth as possible.
"I am proud that our two companies have worked so well together to meet our commitments to customers, communities, employees, and shareholders. We look forward to showing our new customers how KeyBank will help them make better, more confident financial decisions."
KeyCorp and First Niagara also agreed to sell 18 First Niagara branches in the Buffalo region of New York to Northwest Bank, a subsidiary of Northwest Bancshares, to avoid antitrust concerns emerging from the proposed merger.
The deal has already secured the shareholder nod in March 2016, and Federal Reserve approval in July 2016.
Meanwhile, KeyCorp has also agreed to cap its banking layoffs related to the acquisition to 250 in New York.