The Philadelphia Flyers mascot did NOT assault a 13-year-old boy ... according to Philly police, who just announced Gritty has been cleared after a weeks-long probe.
As we previously reported, a man claims his young son was punched in the back by the mascot at Wells Fargo Arena during a Nov. 19 photo shoot fan event.
Chris Greenwell -- the father of the alleged victim -- says Gritty "took a running start" and "punched my son as hard as he could" after he alleges the kid "playfully patted" the mascot on the head.
Greenwell says his son suffered real injuries from the altercation ... and required a trip to the chiropractor for treatment.
But, cops say after they received a report of the alleged assault on Dec. 21 ... they found no evidence saying Gritty committed a crime.
"That investigation, which has been completed and is no longer active, determined that the actions of the individual portraying the Flyers' mascot did not constitute physical assault as alleged," cops said Monday in a statement to TMZ Sports.
The Flyers -- who previously adamantly denied the accusations -- added in a statement, "We are pleased that the Philadelphia Police Department concluded there was no merit to the alleged claim."
The team added, "The police department's statement confirms our thorough internal investigation that found no evidence of the described actions ever having taken place."