The merger will create a combined holding company and a bank that would operate under the name of Huntington.
The companies did not provide a value for the merger, but Huntington expects the deal to add 18% to its earnings per share in 2022.
Softer financial regulations and lower corporate taxes under the Trump administration have emboldened regional U.S. lenders to pursue growth through deals as they compete with bigger players such as JPMorgan Chase & Co and Wells Fargo & Co .
Last month, Spain's BBVA sold its U.S. business to PNC Financial Services Group Inc for $11.6 billion in cash in one of the biggest global bank deals this year.
Huntington Bancshares has a market value of $13.15 billion, while TCF Financial's market capitalization is $5.3 billion, according to Refinitiv data.
The combined company will have about $168 billion in assets, $117 bln in loans and $134 bln in deposits, the companies said, adding the merger was expected to close in the second quarter of next year.
Huntington Chief Executive Officer Stephen Steinour will remain as the chairman, president and CEO of the combined holding company.