كتب : وكالات
A header from Paraguayan Lucas Barrios at the far post bounced off teammate Jakub Blaszczykowski and into the net to put Dortmund in front after 20 minutes.
Lukas Podolski leveled the score ten minutes from time with a powerful shot from the edge of the area.
Sahin, who was earlier provoked by Podolski after the Cologne forward held three fingers up to remind him of Germany's 3-0 victory over Turkey in the Euro 2012 qualifiers, struck the winner in the first minute of the additional time.
He exchanged passes with Robert Lewandowski and found the net from close range to give Dortmund their seventh consecutive victory.
Jurgen Klopp's side, who are missing Egyptian injured striker Mohamed Zidan, are on top of the table with 21 points from eight games.
Surprise package Mainz can regain their place on the top if they maintain their 100% record at the expense of Hamburg on Saturday.