Barnett, 26, was taken into custody for battery after allegedly being involved in a physical altercation about 2 a.m. at the Wet nightclub, according to a police statement. He was taken to Outagamie County Jail in Appleton and released within an hour after posting a $500 cash bond.
Barnett appeared for a charity softball event later Sunday, participating in a preliminary game involving Packers players and sponsors. But he left the field before the regular game and didn't return.
He was unavailable for comment.
In a statement issued after the game, the team said, "We are aware of the incident in Appleton involving Nick Barnett. The Packers organization is going to defer further comment until we have had a chance to speak with Nick and gather more facts."
Barnett is in his fifth year with the Packers, who selected him in the first round of the NFL Draft in 2003. Barnett has been the team's starting middle linebacker since his rookie season.
The Packers rewarded Barnett with a six-year, $34.85 million contract extension in April.
His arrest comes in the midst of the Packers' organized team activities. The voluntary workouts, which started May 30, resume Monday and end Tuesday.
"It's unfortunate," said veteran long snapper Rob Davis, who played in the softball game. "Nick's a good kid. He's been a damn good player for us. I'm just going to wait until the facts come out. I'm not going to think about it."