Diamond, 37, of Port Washington, was charged with three misdemeanors: second-degree recklessly endangering safety; carrying a concealed weapon; and disorderly conduct, use of a dangerous weapon, according to a press release from the police.
He was being held Friday in lieu of $10,000 bond.
According to an arrest report released Friday afternoon, the stabbing took place late Thursday night during an argument in the Grand Avenue Saloon.
Diamond said he and his girlfriend, Amanda Schutz, 27, were inside the bar when a group of intoxicated people started behaving in a rude and insulting manner, leading him to believe "there was going to be a brawl," the report said.
He said he turned to close his account and get their coats, but when he looked back, his girlfriend was bleeding and two men were holding her hair, the report said.
Diamond said he moved toward his girlfriend when a man grabbed him, the report said. Diamond said he he might have stabbed the man with a pen used to sign a credit card receipt, but later conceded he was armed with the knife, the report said.
"Diamond stated that he did not intentionally stab the individual in the bar but that chaos broke out and people were grabbing at him and that he swung his arms to break free," the report stated.
Diamond and Schutz drove away from the bar and were stopped by police, who said they found a "stiletto folding knife with a 3.75-inch blade, which uses a thumb stud and assisted opening mechanism." Police said the tip of the knife was moist.
The man identified as the stabbing victim told police he didn't see a knife or initially feel being cut. Police said the wound was not life threatening.
Other people in the bar said the fracas started when women started shoving each other, the police report said.. One witness told police a woman was upset because people were taking pictures of her and her boyfriend.
Schutz was charged with disorderly conduct and released, police said.
Port Washington is north of Milwaukee on Lake Michigan.
Diamond appeared on "Saved by the Bell" as awkward nerd Samuel "Screech" Powers from 1989 to 1993 and later appeared in various spinoffs of the hit show.
In September, Lifetime aired a movie about life on the set loosely based on a book written by Diamond.
The book garnered negative criticism for Diamond from castmates, who said it was inaccurate.
In 2013, Diamond distanced himself from the book, telling OWN that the book had been written by a ghostwriter and was a "disappointment" to him.