Ferrero, the European confectionery giant behind Nutella and Ferrero Rocher, is set to acquire WK Kellogg Co ., the maker of Froot Loops and other classic American cereals, in a $3.1 billion deal, according to a press release issued Thursday.

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...