Anthony Weiner deserves "no mercy" and got exactly what he deserved ... according to at least one "concerned New Yorker," who took the time to write to the judge in Weiner's case.

An anonymous citizen wrote a letter, obtained by TMZ, to Judge Denise Cote last week, and begged her to see him as a "perverted and deviant sexual predator whom in my opinion should be shown no mercy by the court."

The concerned citizen objected to Weiner's attorney portraying him as a "sick man, incapable of distinguishing rights from wrong and having no control over his own behavior." The angry NYer insisted Weiner is a "danger to our society" and added, showing him leniency would send the wrong message that even disgraced elected officials get a pass in court.

The final shot ... the citizen described the ex-U.S. Congressman as an "average performer with little care for his constituency, while hiding his dark and perverse secrets."

To be fair, another citizen wrote the judge and asked her to go easy on Weiner because he deserves a "pathway to righteousness and a chance to rebuild his life." He suggested house arrest would be best.

Judge Cote clearly disagreed.