The 21-year-old full-back has risen through the ranks at Borussia Park and has become a prominent member of the first-team set-up this season, making 13 Bundesliga appearances thus far.
Korb grasped his opportunity when regular right-back Tony Jantschke was required to fill in at centre-back and he has since been able to maintain his position in the starting XI.
"Julian has managed to make it into the starting 11 this season," he told the club's official website.
"We are pleased that we brought in him another player from our youth team into the first team, and could tie him to us for the long-term."
Despite an eight-match winless run in the Bundesliga, Lucien Favre's men remain in the hunt for a UEFA Champions League berth in seventh position, just five points adrift of fourth-placed Schalke.
