Shaquille O'Neal -- Under Investigation ... Allegedly Punched Man at TV Studio

Shaquille O'Neal is under investigation by Atlanta Police ... after a man claims the ex-NBA star punched him at Turner studios last year ... though Shaq claims the two were simply horsing around.

According to the police report, obtained by TMZ Sports ... 44-year-old Robert Williams claims he was working at Turner Studios, where Shaq tapes "Inside the NBA" ... when he was attacked by the big man just after midnight.

"While at work at Turner Studios on May 9th 2013, about 12:30am, as I was performing my duties at work, Shaquille O'Neal ran out of a room and hit me with a tackle punch to the back knocking me down and falling on top of me injuring my neck and back."

Williams says he reported the incident to Turner Security and was placed under medical care ... though he did not file the police report until July 2, nearly two months later.

As for Shaq, he claims the whole thing is being blown out of proportion -- his attorney told WSBTV, "This was nothing more than horseplay. No one intended for anyone to get injured."

Cops tell TMZ Sports no charges have been filed against Shaq and reinforce that Shaq was never arrested ... but the matter is still under investigation.

7:59 AM PT -- The accuser, Rob Williams, apparently worked as a camera operator on "Inside the NBA."