Lawyers for Sean "Diddy" Combs are arguing his sentence is far too harsh ... accusing the judge in a new legal filing of punishing him for crimes for which he was acquitted.

The rapper's legal team filed an appeal Friday in which they point out a jury found Diddy not guilty of sex trafficking and RICO charges ... yet Diddy received a sentence his lawyers say is roughly four times higher than normal for his prostitution-related Mann Act convictions.

Diddy's lawyers blast his 50-month sentence as a "perversion of justice."

They say the judge engaged in "acquitted conduct" sentencing -- the practice of increasing punishment based on rejected allegations -- which they insist has been criticized across the legal landscape.

Diddy's team wants an appeals court to order Diddy's "immediate release and grant a judgment of acquittal or at least vacate and remand for resentencing."

As you know ... Diddy was acquitted on the major federal charges he faced last year following a two-month-long trial. He was convicted on two counts of transportation for the purpose of prostitution in July.

Months later, Judge Arun Subramanian sentenced Diddy to 50 months in prison ... for which Diddy's lawyers claim was him acting like a "thirteenth juror."