The Cleveland Police Dept. is NOT investigating Myles Garrett for assault stemming from the helmet attack on Steelers QB Mason Rudolph, a spokesperson tells TMZ Sports.

We asked if the CPD was planning to arrest the 23-year-old and we were told quite simply, "No."

"The person who was hit would have to file a report. There is no criminal complaint filed," Cleveland PD Public Information Officer Sgt. Jennifer Ciaccia tells us.

The incident went down in the final seconds of the game at FirstEnergy Stadium in Cleveland -- when Garrett and Rudolph tussled after a play ... followed by Garrett ripping off Mason's helmet at swinging it at his head.

After the game Garrett spoke about the incident -- "What I did was foolish, and I shouldn’t have allowed myself to slip like that," Garrett said ... "It’s out of character."