The breach was discovered after the credit and debit cards were fraudulently used in Texas between April 22 and June 10, police said.
Local banks traced the fraud back to Conor O'Neills Restaurant in Ann Arbor, and management there contacted police.
It is unclear how many customers' credit and debit cards were accessed, and because the charges took place in other states, Ann Arbor police are unaware of the total number of fraudulent purchases.
Caroline Kaganov, general manager at the bar, told WDIV in Detroit she was surprised by the cyber attack.
“We’re a small, local business and we just don’t imagine being targeted,” she told the TV station.
Ann Arbor police say it will continue investigating the case.