The midfielder, who is on an 18-month loan from the Blancos at Signal Iduna Park, also revealed his intention to become the best player in the Bundesliga once again.
The two teams met in Group D of the competition earlier this season and BVB beat Madrid at home and held the Spanish side to a draw at the Santiago Bernabeu.
However, the midfielder - who is on an 18-month loan at Signal Iduna Park from the Blancos - is anticipating Jose Mourinho's men will up their game due to their desire to secure a 10th European Cup success but still believes Jurgen Klopp's men have a chance.
"Sure, we can beat them again," he told Marca on Wednesday. "Dortmund won in the group stage and can do it again, although Real are a much better team now than it was then. It will be a tough game, because I know what winning the Decima means to them. The club's players and fans want the tenth, but we want to win the Champions League too.
"Both teams want to win. None of our players have ever won the Champions League and on Real Madrid's squad there are only three or four who have, so both teams really want to win. For Real Madrid, the Decima means everything, but it does for us too, because we haven't won the Bundesliga or the DFB-Pokal either."
Sahin also revealed his desire to renew his tag as one of the best players in Germany having spent last season on the fringes of the Madrid team and then the first half of this campaign failing to start for Liverpool on loan.
"What happened in Liverpool was... it was very difficult," the Turkey international added. "This isn't the time to discuss it. Returning to Dortmund was the best thing for me. We're in the Champions League, I know the coach, my teammates, I have my family near me and my wife and children are happy here.
"And coming back here was important for my game too, to feel the feelings I had before I left. My goal is to regain the status of best player in the Bundesliga and I feel that I'm well on the way to doing that."
Despite failing to break into the first team at the Bernabeu under Mourinho, the midfielder insists that he enjoyed his Liga-winning season with the Spanish giant and praised its Portuguese coach.
"It was a bit difficult, unlike anything I'd experienced before," Sahin continued. "It started with a fairly long injury and then I didn't get back in form or on top of my game.
"But I won La Liga with Real Madrid, worked with Mourinho who is a very special coach, and shared a changing room with fantastic players."