The 30-year-old centre-back confirmed the decision on the Spielmacher podcast, revealing that a knee scare during a Bundesliga match at TSG Hoffenheim earlier this season brought him to the moment of clarity he had been approaching for some time.
Borussia Dortmund had already announced in March that Süle's contract would not be renewed when it expires on 30 June.
The trigger came in the away dressing room in Hoffenheim after the team doctor ran a drawer test on his knee — a standard check for cruciate ligament damage.
"What I felt when our team doctor did the drawer test in the dressing room in Hoffenheim, looked at the physio and shook his head — I went into the shower and cried for ten minutes. In that moment, I really thought: it's torn."
An MRI scan the following day confirmed the damage was less severe than feared. But the scare had already settled the question Süle had been asking himself.
"When I received the good news, it was one thousand per cent clear to me that it was over. I couldn't imagine anything worse than looking forward to life — being independent, going on vacation, spending time with my children — but then having to process my third cruciate ligament tear."
Süle made 12 competitive appearances for Dortmund this season, starting eight. He made 49 appearances for the German national team, scoring once, and was part of the squad that reached the 2022 World Cup quarter-finals.
He joined Dortmund on a free transfer from Bayern Munich in 2022, making 109 appearances in four seasons and coming within moments of winning the Bundesliga title in his first year before Bayern snatched the trophy on the final day.
Dortmund sit second in the Bundesliga with two rounds remaining, Bayern having already secured the title.