The Poland international put himself in the shop window with an assured showing at Euro 2016, where he helped his country reach the quarter-finals before being knocked out on penalties by eventual champions Portugal.
Kapustka was particularly impressive in their tournament opener against Northern Ireland, a 1-0 win for the White Eagles, and he went on to make another three appearances at the European Championship in France, including in the shoot-out defeat to the Selecao as a substitute.
The 19-year-old winger has put pen to paper on a five-year contract with the Premier League champions following two seasons in the first team with the Stripes, having been promoted from their youth set-up in 2014.
Kapustka joined the rest of the Foxes squad on Wednesday in Stockholm ahead of their International Champions Cup clash with Barcelona later that same evening and becomes Ranieri's fifth signing of the current transfer window. with Luis Hernandez, Ron-Robert Zieler, Papy Mendy and Ahmed Musa all having joined recently.