The Philadelphia Police Dept. tells TMZ Sports ... it is aware of the incident and has launched an investigation.

"We are asking the victim and anyone who may have knowledge of the incident to come forward," the PPD added.

A New York Yankees fan got so upset with a guy outside of the Pinstripes' game in Philadelphia on Tuesday night ... he unleashed a violent smack that appeared to do some serious damage.

The heated interaction all went down at some point either during or after the Yanks' 7-6 extra-innings win over the Phillies at Citizens Bank Park.

It's unclear exactly how the altercation started, but you can see in video from just outside the stadium, an Aaron Judge supporter got right in the face of a man who was wearing a black shirt and red shorts.

The Judge fan clapped his hands together multiple times, then begged the guy to take a shot at him. He then smacked his own face a few times.

The other individual appeared to try to diffuse the situation -- looking in the opposite direction of the man. But, as he turned his head, the Yankees fan walloped him with a Power Slap-style smack that sent him straight to the ground.

The guy appeared to be knocked out cold -- while the fan in the Judge jersey jetted from the scene.

No word yet on if any arrests were made or on how the man is doing now ... we've reached out to cops, but so far, we haven't heard back.

This marks the second time in a week that a Yankees fan has been involved in a fight at game. Just days ago, a group of NY supporters got into it with some Mets fans in a wild dust-up.