'End of an era': Reus to leave Dortmund at season's end
Both parties agreed Reus' contract would not be extended, although the 34-year-old wants to keep playing and will not retire.
"I've dedicated half my life to this club, experienced a lot of ups and downs, but more ups I think," the Dortmund-born Reus said in a video message.
"The club and I came to the decision not to extend the contract. It made me incredibly proud, I'm incredibly grateful to have played for this club for so many years."
Reus, who came through Dortmund's junior system before returning to the club after breaking through at neighbouring Borussia Moenchengladbach, won two German Cups with the side but never lifted the Bundesliga title.
The forward came closest last season, where Dortmund were atop the table heading into the final day but a 2-2 home draw against Mainz saw the title go to Bayern Munich for the 11th straight time.
While Bayer Leverkusen's breakthrough Bundesliga title means Reus will not lift the shield with the club, his side are two games away from winning the Champions League.
Reus came off the bench in a 1-0 win over Paris Saint-Germain in Wednesday's first semi-final at home, with the return leg taking place next Tuesday.
A win or a draw in Paris will see the side through to the final at Wembley, where Reus played in Dortmund's previous final, a 2-1 loss in 2013 to rivals Bayern.
"We want to go to Wembley, we want to bring the trophy to Dortmund.
"That's why it's simply important that the decision has been made and that the issue is then over. And that we all concentrate on this big goal."
In total, Reus has spent 21 years with Dortmund and was captain of the club from 2018 to 2023.
The two-time Germany player of the year was among the best attacking talents of his generation, but injury robbed him of a chance at World Cup glory in 2014.
He played 48 games for Germany, scoring 15 goals.
Reus reportedly turned down a number of offers from larger clubs in order to stay at Dortmund.
In a statement issued by the club calling the news "the end of an extraordinary era," BVB managing director Hans-Joachim Watzke praised Reus as "one of the club's greatest players" and wished him "all the best for his future.
"We very much hope that he will return to BVB after his professional career, because there are plenty of exciting tasks waiting for him here in Dortmund."