The Bundesliga champions wrapped up the domestic title in record time with a victory over Hertha Berlin in late March, but have since struggled for consistency.
A 5-0 aggregate loss to Real Madrid ended hopes of defending their UEFA Champions League crown, while Bayern's unbeaten league record was also ruined by defeats against Augsburg and Borussia Dortmund.
Jurgen Klopp's men now stand between Pep Guardiola's side and another piece of silverware, and Ribery believes that it will be a defining game.
"If we lose in Berlin, it is not a good season," said the France international. "We need to win against Dortmund, that would make it a fantastic season.
"Playing in the DFB-Pokal final is always special.
"Dortmund are in good form at the moment, we'll have to work hard."
Ribery may be limited to a cameo performance in Berlin after admitting he was "still feeling a little pain" from a recent back injury.
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