The letter, made public today, comes several days after Defence Minister Antoni Macierewicz accused his predecessors of leaving the country defenceless with mismanagement and allowing Russian agents to influence military training.
The signatories say Macieriewicz's claims aren't true.
They said he has discredited the army, undermined the country's image with its allies and encouraged "aggressive activities against our country" by enemies.
A spokesman for the president, an ally of the ruling Law and Justice party, denounced the letter as a "tool of the opposition" being used in a "political fight."