EXCLUSIVE

Dustin Diamond﻿﻿﻿ a.k.a. Screech is behind bars ... after he was arrested in Wisconsin for allegedly stabbing a man with a switchblade -- TMZ has learned.

The Port Washington Police Department says Diamond was arrested early Friday morning for possession of a switchblade, reckless endangerment and carrying a concealed weapon.

Cops tell us Screech allegedly stabbed a guy after getting into an argument late Christmas evening that quickly escalated-- the bar owner says it was over pics they were taking of Diamond -- and then the actor bailed from the bar.

According to court docs ... the victim suffered a 1/2 inch wound under his armpit and bled heavily -- but he didn't notice it had even happened until moments later.

Officers located Dustin and his girlfriend in his car not long after ... and the knife was in his ride. Dustin's girlfriend was booked on a disorderly conduct charge.

Cops tell us the victim suffered non-life threatening wounds ... and he's already home recovering.

Diamond is claiming self defense ... saying he and his girlfriend were attacked in the bar. The actor appeared in court Friday morning and his bail was set at $10K.