The Grammy-winning crooner is a HUGE Saints fan (he was born and raised in New Orleans) -- and while he's usually a chill guy, his blood was BOILING when his team got screwed by the refs in the waning minutes of the game.

So, Connick decided to pen a letter to the NFL boss -- telling the Commish he's mad as hell at the officiating and he ain't gonna take it anymore.

"Until changes are made, specifically, booth review of outcome-altering no-calls, you can count me out," Connick Jr. wrote in his letter.

"I'm not watching the AFC championship game and I won't be watching the Super Bowl."

So far, no response from Goodell.

Here's Harry's letter in full ...

"Dear Commissioner Goodell,

Please know that I would've written this letter regardless of which teams were playing in the NFL championship.

I love the game of football so much -- the NFL gives me great joy and has for many years. To not call the pass interference/helmet-to-helmet penalties at the end of the game was one of the most disgraceful no-calls I have ever seen.

The officiating this year has been, for the most part, mediocre, in my opinion. But for a game as important as the NFC championship, it sickens me to know that the people who are assigned to fairly officiate the game seemingly had no regard for not only a dangerous penalty but one that would certainly have changed the outcome of the game.

Until changes are made, specifically, booth review of outcome-altering no-calls, you can count me out. I'm not watching the AFC championship game and I won't be watching the Super Bowl.

Thank you for taking the time to read my letter.