The state Labor Relations Board ruled this month one officer assigned to the Redgranite Correctional Institution had been shorted 35 minutes of wages per week for eight weeks.
The ruling could mean nearly $5 million in back pay for the entire state prison force of 4,800 officers, the Milwaukee Journal Sentinel reported.
"I am not asking them to give me something that I haven't earned or doesn't belong to me," said Officer Paul Mertz, who had raised the issue and is owed about $1,000 dating back to a change of payroll policy in March 2012.
The Wisconsin Association for Law Enforcement, which represents the state's corrections officers, said it would seek back pay for all of its members.
The Journal Sentinel said the dispute was over whether guards should be considered on duty while taking part in pre-shift inspections and then walking to their post each day. The state wanted to start the clock when the individuals reached their assigned duty station but the Labor Relations Board said although roll calls were considered informal, they do mean the start of the work day and the 5-minute hike Mertz then made to his post should have been paid for.
"He (Mertz) is, despite the absence of an actual time clock, 'on' the clock at the conclusion of roll call and fitness for duty check, and (department policies) cannot change that fact," the ruling said.