The Hungarian forward has signed a three-year deal with the Foxes after his contract with Pompey expired following their relegation from the Championship.
Leicester boss Nigel Pearson was delighted to bring Futacs to King Power Stadium as he looks to gain promotion to the Premier League.
"Marko has only been in English football for a year but has shown all of the physical and mental attributes to be successful at this level and we are delighted to have added him to the squad," Pearson told the club's official website.
"He is another player we faced considerable competition to sign, which only serves to highlight how impressively he performed for Portsmouth last season.
"His height naturally offers some variety to our attacking options, but he is a very gifted player whose technical strengths should not be underestimated."