WK Kellogg is Ferrero’s most substantial acquisition so far, with a market cap of roughly $1.5 billion and a portfolio of household-name cereals including Raisin Bran, Corn Flakes and Apple Jacks. Ferrero also bought Nestlé’s US business and Blue Bunny ice cream in recent years.

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...