EXCLUSIVE

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.

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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.