Kellanova sells many of the former company's most profitable brands, including Pringles, Eggo, Town House, MorningStar Farms and Rice Krispies Treats.
It had net sales of more than $US13 billion in 2023 and has about 23,000 employees.
Mars Inc would pay $83.50 per share in cash, the company said on Wednesday.
The company put the total value of the transaction at $US35.9 billion, including debt.
It is the biggest deal in the sector since JM Smucker bought Hostess for $US5.6 billion in 2023, and among the largest of 2024, coming in second to Exxon Mobil's $US60 billion acquisition of Pioneer Natural Resources.
Mars' purchase of Kellanova is expected to close in the first half of 2025.
Kellanova will become part of Mars Snacking and corporate headquarters will remain in Chicago.
Mars, headquartered in McLean, Virginia, is one of the largest privately held companies in the US.
"The Kellanova brands significantly expand our snacking platform, allowing us to even more effectively meet consumer needs and drive profitable business growth," Andrew Clarke, global president of Mars Snacking, said in a statement.
The other company formed in the Kellogg split, WK Kellogg Co, retained cereal brands such as Raisin Bran, Frosted Flakes and Froot Loops, which have struggled with slowing sales in recent years. It is not involved in the deal.
The acquisition would expand Mars' reach into the salty snack category.
The company owns brands such as Combos and Uncle Ben's, but it is primarily known for its chocolates, sweets and pet food.
Mars makes M&M's, Lifesavers, Juicy Fruit gum and Skittles as well as Pedigree and Royal Canin pet foods, among other products.
Sales of some of those products, such as gum, have sputtered in recent years as snacking habits shift.
The deal helps Mars expand into areas of growth.
It might also help Kellanova at a time when rising prices are squeezing consumers and putting many companies under pressure to put a cap on prices.
Economists say many consumers appear to be returning to pre-pandemic norms, when most companies felt they could not raise prices very much without losing business.
Mars got its start in 1911, when founder Frank Mars started making and selling butter cream candy from his home in Tacoma, Washington.
The company moved to Chicago in 1929 and introduced the Snickers bar the following year.
Shares of Kellanova rose almost eight per cent before the opening bell on Wednesday.
