The Turkey playmaker, 24, arrived at Anfield from Real Madrid in the summer but has struggled for form and failed to force his way into Brendan Rodgers first-team plans.
A statement on Liverpool's official website read: "Sahin arrived at Anfield for a season-long stay in August 2012, but will now spend the rest of the campaign with Borussia Dortmund, the side with whom he won the 2011 German championship."
Sahin will now return to Bundesliga champions Dortmund, where he signed youth terms in 2000 before agreeing a six-year contract to move to the Bernabeu in 2011.
With the likes of Mesut Ozil, Sami Khedira, Xabi Alonso and Luka Modric all ahead of him in the pecking order, Real manager Jose Mourinho authorised a season-long move to the Reds.
But despite erasing lingering doubts over a knee injury with three goals in his 12 games for Liverpool, Sahin has opted to jump ship once again.
He has rejoined Dortmund on loan until the summer of 2014 and is delighted to be back at his former club, saying: "I am happy to be home again.
"My contact with the club officials, players and coaches never stopped in the past year and a half. I hope I can quickly help the team."