The German midfielder confirmed his decision in a letter published on his social media channels, closing a career that took him from German football to the Bundesliga, Champions League and Serie A.
Demme spent the most significant part of his career at RB Leipzig, making 214 appearances for the club between 2014 and 2019 before moving to Napoli.
He joined the Italian club in 2019 and remained there until 2024, winning the Coppa Italia and becoming part of the Napoli side that finally ended the club's 33-year wait for the Serie A title in 2023.
Demme returned to Germany last season with Hertha Berlin, making 15 appearances before deciding that the time had come to end his playing career.
His retirement message reflected on a dream that began when he was a child.
"There comes a time when you have to say goodbye to something that has accompanied you for almost your entire life. And for me, that time has come."
Demme had always wanted to experience football at the highest level and was able to fulfil that ambition.
"As a child, I dreamed of one day playing in the big stadiums. I simply wanted to play football and, in the end, I really lived that dream."
His career brought major victories but also defeats and difficult periods, all of which shaped him.
"Great matches, extraordinary successes, but of course also defeats, setbacks and difficult moments that shaped me and made me the person I am today."
Demme said football eventually became far more than his profession.
"Football was much more than just a profession. It was my life, my passion, my everyday life. It gave me so many special people and unforgettable moments."
The midfielder also thanked everyone who had accompanied him throughout his career, including his family, friends, teammates, coaches and the clubs he represented.
His most successful years came with Leipzig and Napoli, where he became part of a historic team that brought the Scudetto back to Naples.
Now, after 15 appearances for Hertha last season and at the age of 34, Demme has decided to close the chapter.
The boy who dreamed of playing in the biggest stadiums got his wish. Now he is walking away from the game with that dream fulfilled.