3:41 PT -- MLB just announced Santana will be facing punishment for his actions ... with the league handing him a four-game suspension and an undisclosed fine.
He is appealing the ruling, so his suspension won't begin until a decision is made.

Pirates pitcher Dennis Santana tried to smack the hell out of a heckler during his game Thursday -- and the wild altercation was captured on video.

The reliever's outburst went down during the second game of the Pittsburgh vs. Detroit doubleheader ... after Santana says a man sitting near the Pirates' bullpen "crossed the line."

Footage shot by fans near the incident shows Santana pointing out the trash-talker to security, before he leaps up and attempts to hit the man in the face with an open left hand.

Fortunately for all parties, Santana didn't connect. He was then ushered away from the area by his teammate.

Dennis -- who actually pitched in the game following the dust-up -- told reporters after Pittsburgh's 8-4 win the man's commentary stepped out of bounds "a few times."

"He crossed the line, and I'd like to leave it at that," the 29-year-old right-hander said through a translator. "I've never had anything like this happen in my eight years in baseball."