British media reported Leicester paid around 19 million pounds ($24.4 million) for the 22-year-old, who played 30 times for Freiburg last season as they finished 15th in the Bundesliga.
Young Croatian Benkovic has moved from Dinamo Zagreb on a five-year deal for a fee reported to be about 13 million pounds.
The 21-year-old arrives at the King Power Stadium on the back of an impressive 2017-18 campaign with Zagreb in which he made 30 appearances in all competitions and helped them clinch the Croatian league title.
"Filip is an exciting young player who has the potential to develop even more at Leicester," Leicester boss Claude Puel told the club website.
"He has made a big contribution to Dinamo Zagreb over the last three years and has also benefited from experiencing European football."
Soyuncu and Benkovic took Leicester's total of summer signings to seven along with Jonny Evans, James Maddison, Ricardo Pereira, Danny Ward and Rachid Ghezzal.
The defensive pair will link up with their new team mates ahead of Friday's league opener against Manchester United.
In a further boost to Leicester, England striker Jamie Vardy has signed a new four-year contract that will keep him at the club until 2022.
The 31-year-old joined the Foxes from Fleetwood Town in 2012 and has scored 88 goals for the club in all competitions, including 24 in their league title triumph in 2015-16.
($1 = 0.7789 pounds)
(Reporting by Hardik Vyas in Bengaluru,; Editing by Ed Osmond)
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