NFL's Justin Blackmon -- Pleads Guilty to DUI ... After Bombing Sobriety Test

Former NFL star Justin Blackmon has pled guilty to 1 count of misdemeanor DUI ... stemming from a December arrest in Oklahoma where video showed he could barely stand up straight.

As we previously reported, the ex-Jacksonville Jaguars wide receiver was pulled over around 3 AM on December 19th ... and claimed he only had 2 drinks before getting behind the wheel of his Jeep.

Cops weren't buying it ... and the dash cam video obtained by TMZ Sports shows the guy was absolutely hammered.

Now, we've learned Blackmon entered a guilty plea in court Wednesday morning -- and faces serious time behind bars when he's sentenced in June.