The tentative deal will span three years and applies to more than 430 USW members working at the Northshore mine.
"The United Steelworkers have demonstrated their unwavering support of Cleveland-Cliffs throughout the years, and this new labour agreement for Northshore further strengthens our collaborative partnership," said Lourenco Goncalves, chairman, president and chief executive at Cleveland-Cliffs.
"We look forward to working together with our new USW partners at Northshore and continuing to build a strong workforce for our present and future competitiveness."
In late July, Cleveland-Cliffs announced that it had put forward an offer to acquire 100% of the outstanding stock in US Steel. If approved, the merger would create the only "American steel company among the top 10 steelmakers and one of the world's top four outside of China."
"Our proposal has the full support of the United Steelworkers union," said Goncalves in mid-August.
He continued: "This is a testament to our unwavering commitment to our employees -- which would number approximately 40,500 pro forma for the transaction -- as well as to the communities in which we operate. We have proven in our previous M&A transactions our strong track record of significant value creation and our ability to grow the business through the addition of thousands of union jobs."