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Reports of Nagelsmann's demise as Bayern boss first emerged on Friday (AEDT), and confirmation of the 35-year-old's dismissal meant he lasted less than two full seasons at the helm.
The former RB Leipzig chief guided Bayern to the Bundesliga title last season and led it into the quarter-finals of the UEFA Champions League this term, with Manchester City awaiting in that competition.
Bayern went into the international window second in the Bundesliga, a point behind bitter rival Borussia Dortmund ahead of their meeting in Der Klassiker on 2 April (AEDT).
The club has opted to act now, during the international break, and Tuchel is the man it has chosen to take the high-pressure job.
"FC Bayern Munich have released head coach Julian Nagelsmann," a statement read.
"This decision was taken by chief executive Oliver Kahn and board member for sport Hasan Salihamidzic in consultation with club president Herbert Hainer.
"Nagelsmann is to be succeeded by Thomas Tuchel.
"Tuchel will receive a contract until 30 June, 2025 and will supervise squad training for the first time on Monday.
"Along with Nagelsmann, assistant coaches Dino Toppmoller, Benjamin Gluck and Xaver Zembrod have also been released."
A former Chelsea, Borussia Dortmund and Paris Saint-Germain head coach, Tuchel had also been strongly linked with Tottenham Hotspur in recent days. Spurs could part ways with Antonio Conte before the end of the season.
Bayern was a long-term admirer of Tuchel, and he takes over with an opportunity to make a trophy-winning impact.
The first game in charge for Tuchel will be against his old club Dortmund, a potentially pivotal match in the title race.
Nagelsmann is unlikely to be out of work for long, having built a strong reputation during his time with Hoffenheim, Leipzig and Bayern.