Italian confectioner Ferrero, known for brands like Nutella and Kinder, is buying the century-old U.S. cereal company WK Kellogg in a deal valued at approximately $3.1 billion. The Ferrero Group said Thursday it will pay $23 for each Kellogg share.

Ferrero International SA agreed to acquire WK Kellogg Co. for an enterprise value of $3.1 billion, pushing the Italian family-owned candy business further into the lucrative US market.

U.S. sales of the colorfully packaged morning staple have been in a decades-long decline, a trend back in the spotlight with news that Italian confectioner Ferrero Group plans to purchase the American...