The Turkey star left BVB for Real Madrid in 2011, but was never able to showcase his best form at either the Bernabeu, or during a spell at Liverpool, and has now returned home
The Turkey international midfielder failed to make his mark in the Spanish capital and soon left on loan for Liverpool, where he similarly found opportunities hard to come by.
He was due to stay at Anfield for the rest of the season, but has now cut short his stay in the Premier League to return to his boyhood club, with whom he won the 2010-11 Bundesliga.
Speculation rapidly mounted that the 24-year-old would return to Signal Iduna Park, and the club confirmed the signing with a short message on Twitter.
"He's back! Welcome home, Nuri!"
Speaking at the player's unveiling, club director Hans-Joachim Watzke claimed that they were always interested in bringing the player back to the club.
"Back then, we told Real to give us a sign if it was possible to bring Nuri back," he revealed.
"We have taken Nuri on loan for 18 months. He left no doubt about the fact that he wanted to play here again."
Once the loan period is up, BVB will be in pole position to sign the player permanently, sporting director Michael Zorc also confirmed.
"He is here for 18 months, and regarding the option to buy him, we're in the driving seat," he affirmed.
Sahin himself says that he is looking forward to coming home, and that his time abroad has served him well, despite his lack of playing time.
"I realised that as a footballer and a human being, I belong here 100 per cent," the midfielder said. "Talks with Dortmund started a week ago. Then, two days ago, we got the signal [from Madrid].
"I want to help the team. I am still able to play football. I noticed quickly that I only want to play for Dortmund. I am a better player now, even though I didn't get much playing time."
Meanwhile, BVB left-back Marcel Schmelzer has joined Neven Subotic and Sven Bender in extending his contract at the club until 2017.