The 22-year-old Turkey international has signed a five-year deal and will join up with Claude Puel’s side at the King Power Stadium, subject to a work permit being granted.
Soyuncu made 30 appearances for Frieburg last season as they finished 15th in the Bundesliga, and has been capped 15 times for his country.
The former Premier League champions suffered a blow earlier in the transfer window after losing their influential talisman Riyad Mahrez to Manchester City after a protracted saga.
However, they have crucially retained the services of England pair Harry Maguire and Jamie Vardy, both of whom were the subject of interest following a World Cup campaign where they helped England to the semi-finals for the first time this century.
Maguire, in particular, has been the reported focus of a bid from Manchester United in recent weeks, but has opted to remain with the Foxes as they look to recapture the glories of their 2015-16 title-winning campaign.
The arrival of Soyuncu, who began his professional career in 2014 with Turkish club Altınordu, further helps the club bolster its back-line, as he becomes their seventh signing of the transfer window.
His arrival follows that of fellow defender Filip Benkovic, who also joined the Foxes on a five-year deal from Dinamo Zagreb.
The pair will face competition from former Porto right-back Ricardo Pereira and Northern Ireland international Jonny Evans, all of whom have also been recruited to the King Power Stadium during the off-season.
Leicester start their Premier League campaign away to Manchester United at Old Trafford on August 10 before they host newly-promoted Wolves eight days later on August 18.