Police said Barnett was involved in a "physical altercation" and was accused of battery around 2 a.m. at the Wet. He was taken into custody to the Outagamie County Jail in Appleton and released within an hour after posting a $500 cash bond.
Barnett appeared for Brett Favre's 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.
In a statement issued after the game, the Packers 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 2003 draft. He's been the starting middle linebacker since his rookie season.
The Packers gave Barnett with a six-year, $34.85 million contract extension in April.