In a statement given after its AGM on Wednesday, the DFL declared that all 36 +German professional clubs were against the proposed reforms, which would see the Champions League restructured into a three-tiered system from 2024 onwards.
The reforms were initially proposed by the European Club Association (ECA), of which 13 DFL clubs including Bayern Munich and Borussia Dortmund are also members.
They have since been considered by UEFA, despite being met with fierce criticism from Europe's domestic leagues.
"The currently discussed concept of the European Club Association would have unacceptable consequences for the national leagues in Europe and should therefore not be implemented in this form," said DFL CEO Christian Seifert.
"We must not allow the traditional national leagues to become less attractive for millions of people across the continent," he said.
The initial proposals, put forward by the European Club Association president Andrea Agnelli in March, envisaged a single, three-tiered European competition with internal promotion and relegation, effectively making it harder for smaller clubs to break into European competition.
Documents seen by AFP last week revealed UEFA were working on similar reforms, with plans to reshape the Champions League into four pools of eight teams, with only four of the 32 participants qualifying through their national leagues.
Yet Seifert said the DFL were "convinced" that UEFA president Aleksandr Ceferin understood the importance of the domestic leagues, and would show "foresight" in the reform process.
"Any reform of the already very successful European club competitions would have to be satisfactory for all participants, not just a few," said Seifert.
He warned that if the national leagues were to suffer, it would do long term damage to European football.
"That cannot be in the interest of UEFA," he said.