The security guard who busted out the ol' 5 knuckle shuffle on the sidelines of a San Diego Chargers game last year -- while fans recorded him on cell phones -- has pled guilty to committing a lewd act.

The video went crazy viral ... showing the man in an Elite Security uniform pleasuring himself while the Chargers took on the Raiders directly behind him.

Investigators in San Diego were able to identify the dude and charged him with 1 count of committing a lewd act in public. He later faced the judge and was sentenced to 3 years probation and counseling, according to KGTV.

He will NOT have to register as a sex offender. 

The man was fired from his job directly following the incident and reportedly gave up his security guard license.