The 22-year-old Hungarian is the latest player to leave crisis-hit Pompey, who will receive a compensation fee, and has agreed a three-year contract with the Foxes.
Futacs made 30 appearances and scored five times last season after joining Pompey on a free transfer from German side Werder Bremen.
He is Leicester's fourth signing of the summer after Ritchie De Laet, Matty James and Jamie Vardy.
Foxes boss Nigel Pearson told the club's official website: "Marko has only been in English football for a year but has shown all the physical and mental attributes to be successful at this level and we are delighted to have added him to the squad.
"He is another player we faced considerable competition to sign, which only serves to highlight how impressively he performed for Portsmouth last season."